I've put older MPC and LTI Geep shells on at least five (I think) newer chassis. I use command control, though, so all the swaps I've done were using the chassis of newer TMCC/RailSounds-equipped engines. It's quite easy, and it would be just as easy if you were using a newer non-command chassis.
There are a couple of different shell to chassis mounting systems Lionel used over the years, but those can be adapted without any major issues. Without taking time to go into all the details, suffice it to say the adaptions necessary are pretty obvious, and not very difficult to accomplish.
The only type of adaption that takes a little more time is mating a chassis that uses a 4 screw attachment system to a shell with the old front and rear screw attachment. That may require gluing 4 small blocks of plastic or wood to the shell and threading the 4 screws into those, but that's not a big deal. Or epoxy small L shaped brackets to the front and rear of the chassis inside the shell, after drilling a hole through them, and put a screw through the front and rear of the shell into the bracket, which mimics the older mounting system on those types of shells. Whatever combination you find, it's not hard to figure out how to mount the shell to the chassis.
