Look at it this way, (and again, I don't know the numbers)
Say you got a 10k payout and the buy back is 6k, 4k would be enough to swap the chassis again, depending on diy or paying.
But then would the car be 10k as a cat s?
If you got a 15k payout, then 4k would be enough to do the chassis again if need be, and then you have a 15k car for 10k. But then the cat s thing again.
I don't know your numbers nor market price for fancy 90s, but you do know the numbers, so can you just look at worst case and see if the sums make sense? Then, if it's not "worst case" you're doing well?
That would seem easier to work out than what's wrong with it (with any real certainty) right now?