It depends on what you want to spend and what you need. You can put a stock clone in like a harbor freight 420, home depot 420, etc. About 300 you will have to modify the exhaust and make or get a mount from Vegas. There is tons of info here. This is cheapest and is about double the power of an old tired engine, I have one it runs great on 22 tires you can spin in the dirt. You can use the adapter to run the stock clutch but if the clutch is worn out you should replace. You need a pulley on or behind the clutch to use the starter/gen which I would recommend because it charges 15A and for yard work stuff its much easier to operate. The clones only charge 3A, unless you get a special one like the 18A charge home depot is only one I know of. 3A does not charge battery much unless you run it for hour every start and no accessory use.
Next step up is the Vegas 460 it has HD block and cam/rod its a built up 420 and 460 actual size. This has more power and revs, your cart will go much faster top speed. It is basically same family as any other 390/420/440/460 clone and all parts work, there are a lot of performance parts out there for these honda gx390 clones. After that you can go 625 Vegas its a larger engine with more torque, which moves these carts very well. That is why a 212cc will not do well it has much less torque. 600cc range engines are big most are twins other than the 625, but the 625 is low profile and fits in carts easier. After that you go to twins the cheapest is the harbor freight 22hp and better is the vanguard which can be modded for more performance but it costs more. The clones and vanguard are raced a lot, many parts out there. Some twins can pull wheelies if the clutches are set up right. The stock 420 I have will wheelie up a steep hill with two guys on it and valve floats at 31mph but gets there much faster than stock. The 420 with 460 parts in it and head mods I hit 41mph gps on a road, it has more power than stock 420 at higher rpm. The stock 420 is just like a stock G9 it just has about twice the get up and go. It takes work to make the exhaust quiet though.
He jumped the gun in the race but watch this and see how the clone 420 all stock just pulls away no matter. Green cart is stock 420 other than jetting and exhaust/airbox to fit in the cart. Camera cart is stock G9 with modded airbox and jetted for it, both have no governors. I hit gas only few second after he did but stock cart is quieter, 420 just takes off that much quicker. Note G2/G9 are same cart with detail differences.
https://www.youtube.com/watch?v=y55ouqdV5Rs