After much decision and more run-around and dodging answers, I have reached a decision.
A little while ago, I drafted a letter stating the following:
1. The agreed upon work
2. All money paid so far
3. The various agreed missed deadlines
4. That I will be coming at 9:00AM EDT on Monday, Dec. 20th to retrieve the car. I stated that if the car is operational, I will pay any remaining balance, and that if the car is not operational I am prepared to retrieve the car in as-is condition.
5. If I do need to tow the car, I stated that I will pursue legal recourse to have the car fixed properly at a shop of my choice
I closed the letter by stating that I have faith in the quality of their work and wish for this to have an amicable resolution so that we can preserve the existing relationship.
A friend is currently reviewing it, and if all looks good I will mail it in the morning. I will mail it certified mail, restricted delivery(only the addressee can sign for it, so I know it arrives in his hands), return receipt. Of course I will be following through on that. I have spoken with someone else with a flatbed trailer who will accompany me to the shop on that date.
Hopefully this course of action provokes completion of the car. If not, I'll get it to my parents' house and assess what to do from there. If it's truly a "tuning problem" as they've maintained in the most recent concrete communication, I can't imagine not being able to sort it myself. There's only so much that can go wrong with a car this simple after all. If not, I'll evaluate what to do. I might rent a U-Haul, flatbed it home with me, then pull the lump and evaluate it myself or have a reputable MG engine builder go through it.