I definitely understand the thought of just buying a suitable replacement so that I don’t have to swap out everything. But what would be the fun in that? If this wasn’t a gift then my vision wouldn’t be worth it. I just can’t pad up on this opportunity though. I have to make this build crazy build.You're probably better off just getting a Kraton, Talion Typhon or something.
What I can think of is, you would need to swap the shocks and towers, control arms, drive shafts, center spool for diff, tire/wheels, body and attachments. Pretty much everything but the chassis pan and the motor mount would need to go. If you did swap all the stuff and you are bashing, then you will probably want to upgrade the chassis pan and bracing... Eventually it's a new car.
Now if you want a rally inspired build throw some wheels and a body on it and go play in the dirt. I wouldn't jump it too much in that form though.