Think mine are 22's, and have 39t sprocket. with 200lbs I don't have issue going 30mph. But that's about max, unless going down hill. Son can get 33mph I think. I think with some more tuning, you should be able to hit close to that. Do have upgraded clutch from BD. But not sure that will change top speed that much.
Might think about go 175cc route. One I got from BD, was very nice improvement. Can peel out, on the hardest of dirt. With slight incline, can even get front tires to lift a little.
It's a true plug and play. Save you a lot of head aches. Can buy the full engine from them, or they got everything you can buy from them, and you upgrade your self. save about $300 that way, if I recall correctly.
Even thou very cool idea to have much bigger engine, with a trani with gears. Think you'd half to do alot more reinforcement to entire buggy. Remember, yerfs are gokart. Why they so light. My yerf has been rolled 3 times. All under 20mph. Thou with stood the rolls. Something did give out. And always needed repair. Roll one say at 40 or 50mph. I'm not sure how that would end. But know my kids wouldn't be getting in it alone.
. Still I'd like to put in a fl Honda 350 2 stroke in a yerf. Just see what it would do. But no longer a kids ride.