I have one of those chinese bicycle engines that you see all over the place. In my home town of Guelph, I see one almost every day. You see them sold all over the place: online, the back of Popular Mechanics, eBay. I got mine from a Canadian company called ZoomBicycles a few years back. They are all the same engines.
If you want a fun way to give new life to an old bicycle, one of these engines is a great way to do so. That said, if you are not mechanically inclined and/or have no interest in learning, these are not for you. I've spent about 1 hour riding my motorbike, and about 24 fixing it.
You see, these little two-stroke engines are beautifully engineered. Unfortunately they are made with poor materials, which is not surprising since you can find them for about $100 for a complete kit if you shop around.
Pro tip: In addition to the kit, buy a spare engine, carb, gasket kit, 10+ clutch pad sets, spare clutch and drive gears. They can take weeks to order from China (some are available in North America).
I won't go into detail about the installation. It takes a few hours, the instructions are pretty good, and unless you're missing any major parts, they usually run the first time.
One recommendation though: the manual recommends a fuel:oil mix of 16:1 for the first two tanks. I know where they are coming from, because you want lots of lubrication while the rings wear in; however at 16:1 combustion is far from optimal, and yoy may spend lots of time try to tune a poorly running engine when it is actually fine.
Pro tip: Use a fuel:oil ratio of 20-25:1 for the break-in period.
One day I got the bike running and decided to drive it home from the office, about 7km, or 15 minutes on the bike. After about 1km I lost power and heard angry noises coming from the engine. I headed back to the office and took the gear cover off. Wow. A gear had chipped a tooth, which caused a chain reaction that caused teeth on both gears to chip. Then a piece of shrapnel left the engine, via the engine wall. It tore a large chunk out of the gearbox and its cover.
Above: Note the missing teeth.
Below: That hole is not supposed to be there.
The aluminium that these parts are cast from is quite brittle.