It is not 50cc's but it should have enough power. If not you can get a tail pipe to add some power. This is really easy to pop on to your bike and is about as good a system as you will find.
It seems that if you want an already built bike you are trying to avoid a lot of mechanical work. I would avoid a Chinese 2 stroke if that is the case (those seem to require a lot of time and effort to keep working). If you get a Robin Subaru, Honda or Tanaka you will not have that problem.