2004 Tomos LX Moped

I had this moped for a few years but haven't ridden it in ages. It has a new engine. The only problem is that I don't know why the pedals keep spinning backwards, and the back wheel locks up. I'm sure it's a quick and cheap fix. I converted it into an Ironman-style bike. It's a two-stroke and gets about 90 miles per gallon. I only have the bill of sale from the previous owner. He also fixes them.