Clone, a cheap China knock off of a Honda that needs allot of aftermarket parts and blue printing usually used for oval dirt racing. If you like to tinker and modify engines and have lots of money this might be for you.
Flathead, Briggs Raptor out of production engine some places still race. Needs lots of blueprinting and aftermarket parts. Selling on the used market for $100 - $200. Getting hard to find parts.
LO206, Briggs purpose built kart engine, good price, very low maintenance and most popular right now, Most tracks have a class for this. no blueprinting needed has a sealed bottom end so no cheating possible, lasts forever between rebuilds. If there is a class for this at your local track you want to have one.
Sundog