You can always get an older 2- or 3-wire vex motors from Ebay for less than $10 apiece.
They will perfectly match the dimensions of the mechanical system that you already have.
They could be driven either by the old Cortex microcontroller or you can use any hobby microcontroller that is capable of generating servo compatible PWM signal. Here is an example of robot that we were controlling with ESP8266.
Just be aware that that really old 3-wire vex motors are less powerful, but have built in motor controller and could be connected directly to the microprocessor output just like an ordinary 3-wire servo.
Vex 2-wire motors (393’s) are more powerful and require external motor controller. Vex may still have MC29 motor controllers in stock, which at $10.99 are not that expensive for the power they could handle.
Otherwise you could get a third party motor controller driver boards from Ebay or Amazon. Just make sure they have thermal and short circuit protection and are rated for at least 5-7A to handle the stall current of vex motors (4A thermal shutdown takes a while to kick in).