Here is a frame that can be powered by components from a Z Scale (1:220) Rokuhan Shorty Chassis B. Using this will allow the shorty's motor to fit into smaller, tighter, train shells than the original shorty frame. Many types of Z scale locomotives can be built using this as a starting point. The Shorty must be torn apart to use the motor, trucks, and circuit board in this frame. This is a slightly modified version of the SW1200 frame available in my shop.
Z Scale Rokuhan Shorty Chassis B can be purchased anywhere Z Scale trains are sold.
Instructions to break down the shorty are here
To assemble a powered chassis using this frame, follow the first 16 steps here