Basically, you need:
1) take out all nitro stuff
2) buy / make motor mount (eg: teknorc.com, rc-monster.com, novak etc...)
3) buy / make battery tray (same as above)
4) install electric motor, ESC & battery (1/8 system: speedpassion, hobbywing, novak or mamba)
For monster trucks, better to run 6s Lipo with a 1500kv motor.