awesome truck right there! those X-factors are awesome, imo should go with like an old square body dodge or chevy body, but nowadays alot of it depends on what you can find that fits the wheelbase and width, excited to see what you decide to go with.
regarding engine i would go with at least a .25/.26 or .28, dynamite has a good variety of options, as well as "SH engines" (dynamite uses SH engines as a base). from the Dynamite .28RTR dynamite Big red, or even the .26 Truggy, you will need eithe roto start or pullstart though for that x-factor platform.
my vote would be to get a good new 2.4ghz RX/TX combo, but maybe hang on to the existing to have the original.