I would like to stay with this frame if its at all possible, so If I can use a 80cc engine thats preferable. Im not sure i'll even be able to mount a 49cc on it though. I would want like to find one with the smallest footprint possible.
Also to answer your question. The 3 speed engine is a 80CC engine with a centrifugal clutch for three separate speeds. You can find it here 80cc 3 Speed Motorized Gas Bicycle Engine Kit*::*Bicycle Motor*::*TheLaShop