A "quality" China two stroke (is that an oxymoron?) would get you started. Just buy the kit and install it. This would at least get you running. You can always upgrade parts as you go. However, with a China two stroke you will most likely be forced to upgrade/replace parts after a little while when they start breaking. Here are a few links to start. There are cheaper places, but these guys would help you out if you had questions.
Grubee 48cc Silver Sky Hawk Angle Fire Bicycle Engine Kit