Let me clarify my question.
I have a LEGO model that contains several 9V Mindstorm motors. I want to be able to control their speed using an R/C transmitter. The element that usually connects the output of the receiver to the motor is an electronic speed controller (ESC). Common ESCs that are used in R/C airplanes, cars etc. are very large and mostly incompatible with the LEGO motor voltage.
What I'm actually looking for is an ESC that is suitable to drive LEGO motors, and it should also be reversible.
Thanks,
Avishay