There are several other sites that specailize in mud motors. Not a lot of those here with all our wooden boats.
You typically wont save much money buying a new long trail frame and a new motor and bolting them up. The builders get their motors at a discount and pass the savings on to the buyer.
If you already have a motor then check with Beavertail for a long tail frame to match. Mudbuddy sometimes sells frames, but they are so busy with their newer motors that they may not be able to help.
If you want a short tail you are out of luck in buying just the frame.
Also if you don't buy a complete motor you wont get a warranty on the frame or the motor - both will be void by the manufactures.