There are a couple of things worth mentioning here that no-one has mentioned so far, or require further comment:
1) You're going to need some fabrication skills and tools. about the only thing that is going to simply bolt in place is another chinese 150. Pretty near anything else will require fabrication and driveline modification to fit.
2) While a big block industrial single and CVT is a good combination, it can be pricey. Do not lose sight of the fact that a Comet 40 or equivalent wil also require the installation of a jackshaft and all the associated hardware and componentry. It is a very heavy option, too.
3)Motorcycle engines- Do not have to be of a large displacement- remember, this buggy shipped with a 150. Anything is an improvement. Whoever here suggested a 750- are you nuts? Somewhere between 75-100hp in a little chinese buggy? Not smart! Too heavy, too dangerous. The frame, running gear, brakes, steering components etc, are NOT up to the task. Also- Bike engines do not necessarily need to be expensive. With a little luck and some smarts, a bike engine can end up cheaper than an industrial engine and CVT. Shop the classifieds for a an appropriate donor bike and strip off the engine. Re-sell the remaining parts to recoup money.
4) Getting back to the fabrication thing- If a bunch of fabrication is going to prove to be a problem, you may well be better off to scour EvilBay (or whatever) for a suitable replacement 150cc.
5) Somebody mentioned a 40/41 pitch sprocket on a Comet 40- The Comet 40 does not have a "sprocket pitch". It requires you to buy/build a jackshaft. Put whatever sprocket you want on there. He//, you could use a BELT output if it meets your needs...
6) Lastly- re-shaping a tapered shaft with an angle grinder- Only if you WANT to ruin your crankshaft. Yes, it can be done successfully, NO, you probably WON'T pull it off. Bad advice, IMHO.