Re: If you had $300 and a mountain bike...
GEBE is golden eagle bike engines - their kit is a rackmount powering the rear wheel though a gear/belt/sheave arrangement.
You can get the mount on its own or with engine.
If you know what you are doing mechanically the CIF engines with shifter kit is probably the way to go.. although it might be worth playing with the ratios on a deraillier since they are geared for pedalling not powered operation - sourcing relevant ratios with a suitable step should make for a fast machine.
hope that helped
"....Look, no pedals... *thud*..." The mating call of the motorized-biker.
"RetroEagle" - 32cc Two-Stroke, Piped. 35mph on #13 gear. Reversed stem with springer fork.