Just some random thoughts...
Do you have any of the info you have presented here in writing signed by both of you? As in a contract?
If you pick up your car now then you will be better off. I agree do not give him a heads up just show up with trailer and a couple helpers.
IF you have a contract and IF in the contract you have a complete by date and that date has passed, you might be able to small claims court him to refund the value of the work not completed.
But...
What ever you do get the car.
If he goes belly up and the car is on the property it will be hell to get your property back.
If he rents the property the owner might have the rights to all property left behind, then you have to prove you own the property
