The Porter Cable 7301/7310 trimmer motor has been discontinued and is no longer available. You may be able to have motors rebuilt at a DeWalt Service Center or small motor repair shop.
The Castle TSM-21 is now built with Bosch motors. Please see part #K21123 for the Castle TSM-21 Conversion Kit. The kit includes a motor carriage and both the Bosch Router & Trimmer motors plus the 3/8" collet.