Here's an interesting engine. I can't tell from the image how wide it is, but a minibike frame may be able to accommodate it.
318cc Rotax Ski Doo Snowmobile Engine Clutch Carb Recoil Has Spark 130 Comp | eBay
318cc Rotax Ski Doo Snowmobile Engine Clutch Carb Recoil Has Spark 130 Comp | eBay