[Nonfractal]

Honestly? 
Just "Don't" !

I have only ever owned 2 front motor cars, A pair of fly vipers. 
Not quick, don't handle well. Even with the factory magnets installed.
Without the mags, they don't work at all. 

Convert to digital? Not a chance!   No space at all for the emitter LED because the motor is right up against the guide.

Bronze bushes for the axles but a plastic brush for the prop shaft..  not good. 

Having dug these out of a box in the shed, I'm looking at them and considering if there is anything that  could be done to make them competitive.
Probably  not (in  any classes at any clubs I go to)