I did a swap to a G-2 Yamaha,I may be able to help some.
Mine is an electric start 16 hp Duromax. So far my upgrades are 36lb valve springs RLV muffler,straight header,drilled main jet removed timing key so advance is about 20' advanced Twin Air filter mounted in dash with 1.25' hose running to it.GPS speed was 37mph but got there very fast. and 23" tires
Now . Your right about the charging system,weak. But I have 2 x 55 watt headlights and 4 X 7watt running lights and running all day charges it up for
night time trips.It truely is not a problem, and besides all these motors come with a pullcord. You will need a new clutch or a insert from one of these vendors as your clutch is tapered and the output shaft is straight. O Stock .com has these motors right now for 338.08 and 2.99 shipping what are you waiting for