I totally agree with Bob f.
You absolutely need to know what speed the engine is turning. My 25/2 governor was set to full RPM at Full Load, so when I started mine on some blocks of dunnage, it started walking and spinning at the same time!
Make your frame first. mount your generator, etc. untill the weight of everything is in place.
Then start your engine at a slow speed. Then slowly increase the speed. Use a photo tachometer to observe the RPM.
You will notice right away that single cylinder engines have smooth RPMs and RPMs that they want to hop around.
Note the smooth RPM range. Mine was about 940 RPM, getting twitchy at 1000 RPM, and WILD at 1005 RPM.
I ended up using a pulley that has my engine running at about 735 RPM, not the smoothest, but very good.
I tried stick on wheel weights inside the flywheel rims, I even cast a big chunck of lead in once. Twins are alot more difficult to ballance. Be glad you got a single.
Anyway, the pulley you select will set the RPM the engine must run to maintain 60 cycles (or 50 cycles depending where you are)
So select a pulley size that has the engine running in it's smooth RPM range. That might be slightly de-rated, but I found that I liked the engine running at 735 better that 1000, and the safety factor (overseed) is welcome too.