My friend had a few broken Scalextric cars lying around (not to mention a lot of non-broken ones) and I had a spare 12v electric motor from a HoneyBee King 2 remote control helicopter. My thoughts began to wander ...
It was fiddly to get the huge motor securely fixed to the chassis and the motor shaft nicely lined up with the drive gear on the rear wheels and the body shell back in place.
Sadly, the car never actually worked. It drew too much current and triggered the short circuit protection on the Scalextric power supply.
I did have plans to sort out some kind of booster / override for the power supply but annoyingly my friend decided that it would be interesting to take a blow torch to the car and photograph the flames melting the plastic. Um, yeah. In fairness, it was my fault for not making it clear how much work it had taken and the potential for one day solving the power supply problem.