Looks like the Chinese clone of the Mitsu engine, commonly found in many of the larger 'superbike' pocketbikes. 49cc, piston port 2-stroke.

Some stock parts for this motor can be found here:
http://www.davesmotors.com/store/49ccparts.html

Adding a good 15mm Walbro WYK carb (like the WYK-58) really wakes this engine up.