Basically, you've got a blank canvas and the sky (and your wallet) is the limit.
If the frame is good, everything else is replaceable. Even if it isn't, frames can be repaired.
First thing is to identify exactly what you've got. Marathons came as resistor carts and with electronic speed controllers.
Next you have to figure out what you want to end up with, which will probably change along the way, so plan ahead. (IE: It's better to overbuild and underuse than it is to underbuild and overuse.)
You've got the makings of a nice vehicle.