This is just one of those things that's so stupid and pointless and yet causes so much trouble... I'm not saying that it's a triviality that you're having engine troubles, because I know I would freak if that happened to me, but the circumstances leading up to this could have been avoided had there been slightly more thought on the behalf of the parties involved... Now you're not only dealing with the issue of the money and liability, but you're also set up for some personal reprecussions as well... Then again, none of this was your fault, so if the guy would just fess up and pay his dues there wouldn't be any problems...
Personally, I don't know if small claims court would work... Unless you have stacks of reciepts showing that you've taken immaculate care of your truck since you purchased it, and can prove that it was in fact the boyfriend that caused this damage, I think you'd be pretty much stuck. Then again, that's just me... you might actually be able to pull it off. I'd try to deal with it personally... I would avoid doing something rash like stealing his car or whatnot, but keep pressure on him to get your money back.
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