We have a guy at the local modular club (
www.hotrak.ca) who does this. He has some beauties...!
He says the key to converting brass locos is to ensure that the motor only receives power from the decoder. So any designs that use the frame, drawbar from the tender, or any other parts of the loco itself to carry electricity have to be modified. Often this simply requires that the motor be remounted on an insulating material (like electrical tape) and fixed using a nylon or plastic machine screw instead of a metal one. Additional wiring may be required to get power to the decoder, and then route it to the motor.
Be aware these are only general suggestions! Your milage may vary! If in doubt, try a well-known source, like Tony's Train Exchange for more info, or to do the work for you.
Welcome to The Gauge!
Andrew