[slotloco]

Hi Ho Lee Cow.
The RX42B was a quicker motor if you can find any.
Also the double shafted 50600, - you just cut the front shaft for 4WD cars off.

I would suggest trolling the Spanish slot shops and Facebook slot car trading groups in Spanish  "slot coches"or "coches de slot" (slot cars)
Remember that in Spain, "SCX" is called Scalextric under the ancient licensing agreement, and what we call Scalextric, is called "Superslot" So for instance, look for "Scalextric RX42B" in Spain

The Flat-6 can be made to fit, but there is no direct adapter, so you would somehow need to screw into the back of the motor through the motor mount are - OR, and it sounds rough, set the motor in place, then hot-glue it, or use a plasticy putty that sets hard, and mould it around the motor.
Only use the yellow can 20,000 RPM Flat-6, it already has more than double the power of those motors you are using  - you may want to increase to a 10 tooth pinion to get more speed, and utilise the much highter torque