A lot of other singer motors will work on a 66. I would put an original Singer motor back on the machine. Motors
from 15-90, and 99 class machines will usually work if you have the mounting bracket with them. There are usually
a lot of 66 motors out there because they made so many machines. Usually degreasing them, polishing the commutator and installing new brushes will rebuild them. Only issue is when the wiring is dried out and the insulation
flakes off. Then they need new wiring.