To use the 15-90 in a treadle cabinet:
1; Remove the motor, (one screw) and light (on screw probably under the rear inspection plate) from the 15, place in zip-lock bag for safe keeping. Make sure you do not loose the motor bracket screw it is not a standard thread.
2; Remove 66 from treadle cabinet. Simply loosen the two grub screws that hold the machine to the hinge shaft. Put the machine in a plastic bag to keep it clean.
3; Put the 15 in the treadle cabinet. Slide the hinge shafts into the two holes on the back of the machine and tighten the grub screws.
4; Put treadle belt on the 15 and adjust to fit.
You might want to acquire a spoked hand wheel for the 15. It will give you some extra needed inertia as you treadle. If you want you can use the one from the 66. The belt guard on the 15 might not work with the spoked hand wheel. The 66 belt guard won't work on the 15.
5; Treadle to your hearts content.
Joe