Yo I vote for calling him up. Think like an attorney and argue that your contract was to get the moped part when you can afford it.

Ask him why he needs it and ask more questions like, "Have you thought of doing such and such?"

If he definately wants to fight tell him you're aware of his cutting someone's eye out and it doesn't sound like a winning proposition to you. Ask him how beating your ass is going to make him give him $50.

Say "Dude, sorry I can't help right now. I would if I could."

Let us know.

Worse comes to worse, post his phone number here and we'll find someone to fuck with him.