You probably will get more love for these on one of the motorized bike forums. There are two and they both like those type of rigs. They banned those type of two strokes in China and now they send them here and sell them cheap cheap. Small 4 cycle engines are available and are a better bet I feel. YMMV
Stick with the fun in the city as transport and forget the offroad ventures. Motorized bicycles can be a valid form of transportation and the best way to get from point a to b in an urban environment but using them for offroad recreation is a whole can of worms that need not be opened. Just use your mtn. bike or get a motorcycle as they are accepted forms already. Just sayin.....
WizWheelz TerraTrike with Velo-Kit & 24V motor, completely enclosed (fiberglass/kevlar & Lexan) EZ SX Tad with 36V motor
May go the same way as pocket-bikes or weed-eaters in this area and be banned outright, but for the time being that power-assist option is still showing up here now and then. With expansion chamber exhaust and some sort of intake booster- that trike would be terrific fun, but illegal since it already goes over the 20 mph limit without lights/signals/license. Noise, smoke, whatever- I like the idea!