If you are going to replace the motor, trucks and chassis, you might as well go ahead and replace the shell as well. A nice Atlas Silver Series SD24 would do the trick.
Simon Modelling CB&Q and Wabash See my slowly evolving layout on my picturetrail site http://www.picturetrail.com/simontrains and our videos at http://www.youtube.com/user/MrCrispybake?feature=mhum
I would look for a Athearn SD-9. Another good choice is a Atlas/ Roco/ Con-Cor SD-24 or SD 35. Both are inexpensive.
If you want to spend more money then an Atlas Master SD 24 or SD 35. A P2K SD-9 might also work.
Be aware you may have to add plastic mounting pads or blocks inside the Tyco. Then drill through the new chassis and use screws to attach the chassis to the body.
You may also have to cut or trim the new chassis or weight slightly.
The place to buy is ebay.
Jim
I"m thinking of re powering my tyco sd24. It still runs but not the greatest. Does anyone know where I could find a complete motor, pickup trucks chassis?