What's wrong with my small engine?

I bought a motorized bike from someone last summer. It was custom built. I believe the engine on it came from a gased remote controlled helicopter. That's what I was told from a friend. I believe I mixed the gas with to much oil last yr and it sluged up and leaked out of the exhaust. It would not go top speed. When I tried it, it kept jerking and wouldn't wanna go faster. I am going to flush the system out and clean the spark plug. Why won't it go top speed? Is it highly possible the engine is to small? The gas also sat in the bike through the whole winter this year. Can anyone please help me? Why won't it go top speed, and what should I do about the gas being left in all winter?