You can get them in the Titan, the Honda and the Huashang and a bucnch of others.

 

2 stroke kits come in with Tanaka 32, 40cc engines, the mitsubitshi tle 33, 42cc, tecumseh tc 300 49cc, gp460 engines and some other chinese engines

 

4 strokes are the honda gx25, gx 35,ghx50  titan engine, robin suburu 25,35cc engines, the huashang and some other chinese engines

 

you get get Friction drive kits which are a engine with a mount and a roller  that presses against the tire propeling the bike forward.

or you can just get a chain drive.

heres sites that sell rack mount kits or friction drives

 

and some other sites.  i have no experiance with these but i heard there just as fun as the frame mounted ones.