Or, you could just hunt around and find a set of back clamp feet and attachments ..... that would make the 66-1 even more unique. Of course I did it backwards, I got the feet and attachments first then hunted up a machine to match.
I am not a collector. Call me an amasser or a if you want to pick on me a hoarder, but I dislike the name "collector". A collector will take the best parts from several machines to make one really nice one. Generally degrading or in many cases ruining the donor machines in the process. Then the collector takes the one he built up with the best parts (which is no longer original by the way) and sells it for outrageous prices. I totally hate that.
Clean up each machine, polish the plated parts with the appropriate polish and make each one look their best. Each has their story. Switching parts around just confounds things to my way of thinking.
I have no problem with borrowing parts from one machine to make another one run while parts are being order or hunted down, then putting the borrowed parts back on their original machine. I've done that quite a bit. But I never switch parts around because they are prettier than the other one.
As for the motors, your 66-4 was originally a treadle machine anyway, ( note the spoked hand wheel) so no matter how nice it is it's not original. But motors are easy to refurbish. I've lost count of those I've rewired and cleaned up. I just finished rewiring my first 15-91 last night. Nothing to it.
Joe