You can slow down a faster motorized unit by using diodes ties together back to back is series with one motor lead. You can not speed up a slow engine. In DCC equipped motorized units it is easier as you can program parameters controlling motors.
My yellow power tender has a cable that attaches to the engine from the tender. If your engine has a connection point in or near the cab, maybe you can get an appropriate one from TrainLi which will synchronize them.