lots of guys on here use snowmobile engines, mostly for the cvt transmission, also because the ones with reverse are typically an electronic reverse, meaning that the engine actually runs backwards. if you are gonna go that route, i'd suggest you look for one that is liquid cooled, air cooled one are designed to stay cool in cold air, in the heat of the summer it will likely overheat.
as for size, bigger is better, my gut always says it needs to be big enough to go fast enough to do something stupid in. lol... that being said, how big of buggy are you planning?
