The 31-15 has a motor boss, I would have thought the 31-20 would too.
Oh! I guess you meant a bracket! Duh! Sorry, I'm slow today.
I wonder what sort of motor is/was available to put on there. That would be a determining factor, but I bet it wouldn't be hard.
Take your time. This machine sat for 30 years like this, it can manage a few more days.
