You won't find too many that have done this because of the reverse issue. The stock G1 engine actually runs backwards to give the cart reverse, there is no actual reverse gear in the transaxle, as stated above. I considered the same swap to a 4 cycle but decided against it because of the extra work and cost involved with the axle swap.
It can be done, however. And if losing reverse doesn't scare you, I'd go with a clone engine. At around 13 hp or more it will run very nice, lots more torque.
If you go with a Yamaha golf cart 4 cycle, I'd go G2-G9 donor, as the frame itself is very similar and working on them will be similar. But if you have a donor cart....why not just use the other cart?
