As long as the potential buyers knows and understand what they are getting and happy with the final price, then all should be good. It is the surprises later that could be a problem. Then I have issues.
I have a buddy that just called me today that is purchasing a beautiful Wimbledon white 1969 Cougar ELIMINATOR. It does not have the original engine, but does have a freshly built 351 Windsor. I told him he might be paying too much for a non-original drive train and that if and when he goes to sell, that will hurt the price. We both agreed, if he loves the car, enjoys and drives the car regularly, then all should be good.
Take care
Cory