Gears turn but piston won't pump

I've looked for an answer to this for a while and if there is one could someone point me in the right direction. . .

I have a 2 stroke engine that at first seemed to be running fine. . . well I thought it was running a little sluggish. After adjusting everything so that it worked fine (meaning I adjusted the clutch and the idle) it worked well for about a week. Now I can't get the engine to turn over. The first problem is that I got the motor off of ebay. . . yeah, I know big mistake. The second problem is who ever installed the piston rings put one of them on wrong so it had snapped (hence the reason it was sluggish). The third problem and I'm not quite sure what to do about this is that the gear in the gear box will turn but the piston will only occasionally pump. If I roll the bike backwards it pumps (most of the time) and if I go forward it does every once in a while. Now occasionally the engine will turn over. . . and the motor will go whether the clutch is on or off. But it won't move the bike unless I lift up the back wheel so there is no pressure. I haven't yet split the engine in half but if that is necessary I would like to know before hand. Any ideas of what could be wrong?

That sounds like a pretty accurate assumption. How would I get the small cog off to check to see if the problem lies around that. It looks like it requires a giant screwdriver. But I'm thinking that it probably is a missing key. . . I'll try it out.

Okay so the key had somehow popped out and gotten a bit munched on by the gears. . . What happens if I try to run the motor with only one piston ring?

Well it pumps and the engine goes with more power than it did with a broken one in it, but will the missing ring eventually cause big problems?