You can decrease the time between "updates" and still keep the averaging of the frequency by using a simple trick.  Each time you get a frequency sample, you add it to the prior sum and subtract out the oldest sample (you have to keep track of all samples).  That way, you have an average of the frequency at every sample.  You have to get the first (n-1) samples before you can start, but after that you can update more often.

You can also eliminate the "divide by n" (for n samples) by multiplying your target by n up front.  So if you average 5 samples, you simple add them up and compare to 250 (for 50 Hz).

If you're measuring time between samples to get frequency, you can eliminate the reciprocals and just work in "period" as well.  For 50 Hz, the period is 20 milli-seconds, or for 5 samples 100 MS.

Short answer:
Yes, bump clearance is mainly concerning compression ratio.  Assuming you have enough clearance that the piston does NOT hit the head or valves when the engine is cold or hot, then the only concern is compression ratio.

Long answer:

The compression ratio is the ratio of the volume of the cylinder, head, piston, etc at bottom dead center (BDC) to the volume at top dead center (TDC).  Increasing the bump clearance will add a small amount of volume to both BDC and TDC volumes, it can be calculated knowing the diameter of the cylinder and the amount of change in bump clearance.  The area of the cylinder is "pi R squared", the change in volume is the change in bump clearance multiplied by the area.

That's the easy part - the hard part is knowing what volume remains in the head, combustion chamber, etc.  One way is to disassemble the engine and measure the volumes with a liquid such as oil. 

The volume at BDC can be estimated by using the stroke of the engine multiplied by cylinder area, which is added to the volume at TDC.

But who wants to do all the math anyway?  That's why a compression gauge is needed.  Decreasing the bump clearance will increase the compression pressure, increasing bump clearance will decrease compression pressure.  All you need is to know what is the proper compression pressure and enough patience to put in the required shims to make it work.  And enough muscle to crank it over consistently while measuring compression pressure...
 

I have a 9n Ford that was 6 volt and I put a 12 volt battery in it.  I changed the starter relay to 12V (no solenoid).  I changed out the generator for a 12V alternator (bought a kit including alternator, proper pulley, bracket, etc.)  At first, I added a resistor in the ignition but it needed a new set of points every year.  So finally I updated it to a 12v coil and electronic ignition.  It has been working this way for several years now.

That's a lot of work and fair bit of change, so to answer your original question, yes you could put a small 12v solar panel on it.  They typically only put out a fraction of an amp, and will work just fine at 6V.  I don't have a good answer as to how long your battery will last, my experience with batteries has been all over the map.  But it works for your friend, it should work for you as well.

There are also some simple charge regulator circuits that could be built with an LM317 and a few components.  I think there's one in the LM317 datasheet, I don't know if you're into building your own or not.  It could be powered from its own solar panel or powered off of your existing 12v system.

Just a wild guess - maybe it's a fine thread?  M8 x 1.0

Lots of good information right here in this thread - I will especially direct your attention to the safety information covered here.  Don't kill yourself or your generator!  Most scopes have the ground lead tied to the metal chassis - that can result in rather unpleasant things happening if you connect the ground lead to anything that isn't grounded.  There's also a couple of links in the 3rd post that still seem to work - I didn't check them for accuracy but just a quick look seemed like they explained things.

Somewhere I saw someone had taken a flywheel and crankshaft out of a gas engine car (junkyard parts).  They cut the end off the crankshaft where the flywheel mounts and bored a hole in the center to mount it on the CS crankshaft.  I think they may have used a taper lock bushing for the mounting, but I don't remember exactly.  Then put the automotive flywheel on this "adapter" that they built.  It also provides a ring gear that you can use for electric start if desired - you just need to mount the starter in the right spot (and make sure it rotates the right direction).

Be aware that a flywheel with teeth is slightly more dangerous than a smooth flywheel!

If you like digital gauges, here is a sample of what's available:

http://www.mpja.com/Digital-Panel-Meters/products/52/

This page has several meters that are powered from the incoming AC, and can measure volts, amps, watts, frequency, etc.

I'd recommend at least two gauges, one for each 120V line.

The "current transformer" is a coil that is placed over one of the hot lines (and is insulated from the wire).  It typically has two wires that connect to the meter, and allow the meter to measure the current in the wire.  To measure voltage, two wires are connected to the meter, one from the neutral wire and one to the hot wire.  These two wires also provide a very small amount of power to the meter.

You'll want the meters that are for AC voltage and current.

And there's a foundry in Texas as well:

http://www.ociw.com/

Oil City Iron Works
They do foundry work, pattern making, molding, cast iron, machining, etc.

They are in Corsicana, which is just South of Dallas.