Note: I did do all of this, but it was late at night and all the pictures would've been horrible quality, so I didn't take any until I was finished.
To get your Gen1 xmod in tip-top shape, start by stripping it down and cleaning it.
1) Clean off the servo and the electronics throughly..... most dirt collects there.
2) After removing the electronics, start sanding the bottom of the main chassis if there are scratches, or just find a new one. Paint the chassis (and anything else thats tan) to your desired color and clear coat it. I chose flat black.
3) Nows the time to stack FETs, add a hotter motor, tight turns mod (resistors), or even add some nice little LED(s) to the car's electronics.
4) Then replace all the broken or worn pieces with new ones or repair them (knuckles, upper front arms, bushings [or bearings], ect.)
5) Finally, reassemble your xmod with care and make sure everything is perfect. Then, add in a new set of rims and refurbish the body, and you are done!
Finished restoration (minus body and wheels):

That little LED on the mini-z motor lights up bright blue when you drive.

bottom of chassis:

Wheel & body combo:

If you follow those steps, you should end up with something like the above. If not, then better (as didn't remove the scratches on the rear chassis plate).
Anyway, good luck restoring your classic xmod! 8)