The old engine would probably have been fine if you had just taken it apart and cleaned it well. I've cleaned engines that have sat for 15+ years and they still run today.

As for getting running - fresh batteries, fresh fuel, and a good cleaning and adjusting should be all that's needed. You can always find the owners manual online at Traxxas.com if you need instructions/diagrams. And don't forget to break-in that new engine.

Edit: get a couple spare glow plugs too, break-in can be hard on them.