That should do it, VP.  Well done.

+1  Just look up the retail price of a Marathon generator head in that size and you'll see the asking price for the whole set is a bargain.  The waveform on a Marathon head is so (low THD) perfect it's typically described as "voluptuous" sine. 

As Gary noted, you'll kill a diesel engine if you have it running for hours without a significant load.  Sometimes that makes for a bad match to an application when an inverter generator or small LPG generator might be a better match.

Thanks for piping in, Butch. I forgot the essential alignment of TDC and counterweight position.  Yikes, that's critical.

Martin, it looks like you've got a machine with no counterweight on the flywheels, which means you need it on the crankshaft.  Better send a picture from the big door so we can see (and learn).  Also- what's the outer diameter of your flywheel?

Maybe 38AC can suggest how to best tune your balance.  I can only guess that I'd apply his same method only getting the two flywheels first balanced to have zero counterweight.  If it was a hopper you'd have the option of adding weight opposite TDC to the flywheels, or if possible, adding it internally to the counterweight.

One guy here many years ago got a zero counterbalance engine-  none internally or externally.  A real Rajkot surprise. 

I do use an AVR on my ST-3 and so does my neighbor on his.  My neighbor uses one similar to the one you show, his purchased on ebay.  They last about 4 years so he keeps a spare on hand.  His ST-3 setup IS using the harmonic winding; in fact his CGG "brand" ST-3 requires it, the stock harmonic setup is grossly over-voltage. I use an AVR of my own design; a hand soldered prototype. I can flip a switch and revert to the stock harmonic regulation.  My St-3 had a very nasty "harmonic hump" so I use the mains for excitation of my homebrew AVR.  I also offered a schematic and PCB design for a simple AVR here many years ago.  A few members made them. The China AVR's are so cheap that even I recommend them.  As I said, you should stick to harmonic regulation for a welder as these cheap AVRs won't handle a low PF load like some welders well, and will regulate at way too low of an true RMS voltage.  I learned this the hard way, though it was not a welder.

I'm happy to share schematics, etc. with a forum member who needs them.  You can't buy parts for what you can buy the finished China product for these days.

Since you have an ST-7 and what is really an 8/1 already,  you have all the makings of a good welder generator.
Balance issues are easy.  I highly recommend 38AC's method.  I used it on my neighbor's 8/1 and will never go back to the Mr X wheel chalking method.  The latter works, but takes much more time.  38AC first corrects the counterbalance offset angles to match the key location, then adds weight to the lighter of the two wheel's counterbalance weight to make have identical on-axis counterbalance weight as measured by string can and weights..  Then you just add equal weight to each wheel either at or opposite the counterbalance.  Hoppers need more, fore and aft movement or sliders need less. 1 oz per each wheel increments work OK.  It's can be done with only a handful starts/stops.  My hat's off to 38AC for this simple, powerful method.  I used a short, leveled section of hot rolled angle iron on wood blocks and a 12" section of 2" OD axle from Amazon for doing the measuring and matching. Plus a can, string and lead shot.

I would rather not limit myself to 2300 watts continuous for welding if I already had an 8/1 and ST-7!
The faster rpm of the 8/1 does seem a bit "nervous" for a while but it grows on you.

Best Wishe

The typical ST head THD is pretty high from waveform distortions (ratcheting from non-skewed rotor windings, harmonic hump from harmonic winding) will have no adverse affect on your welder.  All the new designs are switch mode power supplies, so the first thing they do is create bulk DC from the AC input.  The cheaper welders with bad power factor (direct rectification of the mains to bulk capacitors may do this in a manner that has lousy power factor.  That will affect an electronic regulator, if you use one on your ST-7, and it is a typical cheapy that doesn't do true RMS voltage regulation...causing it to regulate voltage too low.  I've experienced this situation on my ST-3 with electronic regulation.  On your stock harmonic winding, it will regulate fairly well with a rude, low PF load.  i switchover my ST-3 to the backup harmonic for this situation. 

The usual 10 Hz Listerflicker variation in voltage and frequency will normally be handled quite well by a decent electronic welder.

The big issue I see you having is that you will have to limit your welding current pretty seriously.  If it's a small MIG welder, with output at 24VDC at the machine, you should get 85-90 amps of welding current (allowing for losses), but you're at the limits of the 6/1 power (2300W generated power)  If you can live with that, you should be OK.  I have had a smaller amp MIG welder running on 230VAC, on my 6/1-ST-3 with harmonic regulation. It was fine for thinwall square tube welding.

For moderate current stick or other higher power welding, you don't have the power for 1/8 rods of 7018.  You could barely do 5/32 6013 rods. If your duty cycle was low enough, and you had SOM type heavy flywheels, that would probably help.  An 8/1 would be a better match for home/farm welding. 

PS I've got some heavy flywheels I'd sell at cost if you get serious about doing an 8/1 conversion with an aluminum piston and have the typical 6/1 spoke flywheels.  I don't think anyone thinks they should be spun higher than 650rpm. 

Best Wishes,
Bruce

+1 with admiration for a first quality setup...including the Marathon head. 
One of most brilliant analog and digital circuit designers I ever met was a former Navy electronics technician. I learned a great deal from him.  I hope you find a good home for your Dad's Lister/Marathon generator setup, Schwaps.  It's a beauty.

A most impressive analysis and outstanding explanation, 38AC. You are an excellent instructor and writer.  I learned a lot, thank you!  Thanks to silvertop too.