Sorry, I hope I'm understanding you correctly and can help. Element we would call a fuel-filter?

I am not sure what "misfiring"? Can you hear the injector (nozzle) creaking when the piston is after TDC?
(TDC Top Dead Centre on firing stroke)

The engine will certainly "hunt" (raise and lower itself) if the timing is off, but I would suspect the linkage/govenor first.
Since you say the gear on the camshaft has been altered, that would be the first thing to check. There should be punch marks on the gear teeth that need to align, and a Spill-timing mark on the flywheel. 18°btdc if memory serves me.

I would check this is correct first, before loading the engine, but some engines may hunt at idle, and come alright when under load.

Crankshaft "moving in and out" we might call end float? This would be an issue in the main bearings and will only cause problems when run at speed and temperature.

I used to just buy an engine's worth of cheap oil, drain the old (when hot), fill with the cheap stuff and run till hot, then change filters, drain and fill with good oil.

Now I just drain the old and fill with good oil.

thanks mihit - yes we are in the Uk, hence 230 VAC

what you say about double converting energy makes sense - however, ideally I'd like to pour a concrete base and build a shed around the engine just to try and keep the noise down, it going to be easier to keep the engine "isolated"

one of my reasons for wanting to stay direct drive was just for the simplicity and "compactness", also, even if I could weld, living off grid makes it difficult to get power to weld! so I'd like to avoid having to make a frame to mount pulls and belts etc

thanks for the info on sizing the gen head

Fair point and obviously you know what's going to work best for you. Keep us updated and post pics if you can, we always like pics

Some injectors have a feeler pin which you put your finger or thumb on while it's in engine to check function.
They should definitely go off with a "creak"

As to the original question, a belt each side is almost preferable, balancing loads on the main crank bearing.

I'm not sure what specifications? If it's intact and one of the bryce/ bosch types then it's pretty much clean-up and run it.
I do have an exploded diagram on the PC but don't recall mention of crack pressure or anything. Either measure/mark current adjustment or don't tutu with it!

As for pintle sizes angles etc, if it's that far gone you'll be better finding a good one IMO

OK, Well I'll just go against the grain and say, since your engine is rotating, power anything that rotates off that, via belts etc. You can shield/guard the belts, or just use the safety feature between your ears and not put yourself in harms way.

Next best/safest source is pnumatic/compressed air.
Depending what engine you get, you could pull the injector/pump to one cylinder and run that as a compressor - alternatively you can probaby pick up a belt-drive compressor head cheaply as they seem to be everywhere now.
There is a disadvantage in double or triple converting energy. For example an engine, reciprocating motion, giving rotation, to generate electricity, to rotate a motor, to drive a piston compressor (reciprocating again!)... Waste/conversion loss at every step.

If you can find an old Miller "roughneck" weld/genhead, that takes care of those two things in one go.

The advantage with pnuematic tools is that they'll generally be field servicable. If a rotor or stator burns out, it's a trip to town to order a part to wait a month...