I've had ideas for one of those plastic models.
Steering; get one of the ones that have the moveable steering wheel and attach servo using some sort of mount.
Motor; Make a solid single axle in the back and melt (or something) a spur gear to the axle and make a motor mount, bolt it to the chassis and PROFIT!!!
Battery; Hollow out the engine compartment and use velcro.
ESC; Mount in passenger seat.