To value a car, you have to work back from the end product. If you want the car to keep as a good runner, start at say $25,000 to $30,000 for the finished car value, and work out how much it will cost you to get there, i.e. interior trim, respray and engine build being the most expensive.
Or if you only want the car to break for parts, add up what you can get for the parts separately, and then discount it at say 60% or 70% of the total.