Ok, but if you're going to build something, make it something worthwhile.
Stuffing a YZ250 engine into a CR250 frame is really only a benefit to someone who motocrosses full-time and is good at it to boot. That combination has no real benefits to the occasional trail rider. At least as I see it. The shortcomings of the Honda can be dealt with a lot easier via porting and tuning for the type of riding we do.
So, go buy yourself a 2002 CR250 and have Eric Gorr work his magic on it. Problem #1, selection of a dirt bike, has been solved. Add in a Keihin PWK carb and a flywheel weight and you're good to go on the dirt bike front.
But we've yet to solve the "Dad Project" issue and a Plymouth GTX takes up a LOT of garage space. I'd offer this solution: Pick up another 2002 CR250 and a 1991 & up CR500. Sell the 250 engine on **** and everything non-engine from the 500 on there as well. Give the whole mess to Dad and tell him, "put this engine in that frame." If Dad can weld aluminum, you're in, other than getting the new frame bits machined (unless he can do that too). The end result is a product WAY more fun than the GTX.
I wouldn't worry about the handling, power, or anything else. Any late-model dirt bike is a fine machine and will be plenty of fun, have plenty of power, and handle well enough for whatever you'll be doing with it. I know I'm the limiting factor on all my bikes handling prowess, including the 1982 Maico 490.