I'd pass if it were my money.
I have enough receipts and time slips to sell about 5 cars
I find it odd that he knows so little about the car and has a convenient excuse for why it doesn't run strong now. Most racers could tell you about every bolt in their car. It has mediocre heads, mystery internals, and a bunch of promises.
Just my opinion, only you can decide what it's worth to you.