Send him a written bill via certified mail for the original agreed on amount of $200.
I might even even go buy a $5 book of invoices just so it looks professional.
Write on the bill that the engine will be returned when payment is rendered.
Keep a copy and the mail receipt.
That way, if the cops or a judge ever gets involved, you have evidence to submit with dates and specifics.
Lots of courts will favor those who have an account and the better quality of the account, the more favorable- IE video supersedes pictures, which supersedes writing which supersedes word of mouth, etc
