Olican - if you put that huge engine in an ATV you will be walking bow legged and look like a toy cowboy.....
Hillkiller - You're biggest problem is going to be power. If your Rincon is older, the sub-trans dogs could get rounded off (early problem that was fixed - will not shift between D-N-R, will just grind). This happens when shifting between D-N-R with higher engine rpms. Granted if you have the 400ex shifter on it, this will not cause you problems since you will be able to force it in. Mine had this problem, but I never knew until I put the original shift lever back on it. It is about 1/2 day job to update, but once fixed, you never have the problem again. Sometimes the older Rincons will have the stator go out. You will get low spark but won't run. Basically, the Rincons are solid even racing. Change oil, keep the air filter clean, grease/maintain all pivot points and they will last a long time.
I always wanted to build a Rincon engine. I believe Thumper Racing had a 750 kit. I still have a whole 680 engine that I was going to use, but never did.