Yes. Keep the car (or take it back with your spare key) & sue him TODAY. If you repo the car, leave a note telling him you've done so to keep this all aboveboard. I hope you kept your paperwork & had your agreement in writing! If not, you'll have to try to prove your arrangement by showing the judge your cancelled checks. If you sue in small claims, sue for every penny you've paid towards the car. $2000 + all payments. If that amount takes you over the small claims court limit, sue in district court for the full amount OR possession of the car (if it's still in good shape).
As long as you have physical possession of the car, he can't sell it to spite you. You might want to spend $100 to change the locks to prevent him from taking the car right back. You have a legitimate claim of right or title to the car, so you can't be charged with theft. Actually, you can be charged, but you can't be convicted on the facts you've presented here.
You should get either all the money you paid back or the car. Even if you only get your money refunded, X may decide he'll give you the car back rather than try to come up with all the money right away.
------------------
This is not legal advice and you are not my client. Double check everything with your own attorney and your state's laws.
