I doubt the "hotter" motors fitted in these later cars will be any better, because although they are all rated at around 16-18k, SCX motors are designed to run on about 16volts, Scaly digital produces about 10-11volts at the rails depending on your setup so they just don't hack it.
Slot.It still make an adapter (part no SISP08) so that you can fit a mabuchi motor into an SCX car, unfortunately the pinion shaft of a Mabuchi Motor is not long enough to engage in the crown gear so you will need an Anglewinder axle stopper or spacers to keep the axle/crown gear in the correct place. Apologies as already stated by screwneck
I have found that a Scaleauto Yellow Can (25k) is a good motor for this application provided you fit the ferritman to the motor end of the leads. They are also fairly cheap in comparison to the more mainstream offerings which I have found to be a bit too demanding on the standard Scaly chips.
Regardless of what motors you end up fitting don't waste money on Magnets! fit some decent tyres instead, P6s work best on Scaly Track
Hope that helps
Regards
Steve