roguequilter, from what I've read those Japanese motors are interchangeable, IOW. one from a healthier different Japanese SM might keep your machine alive.
Maybe someone with practical experience in switching motors can chime in...?
As far as being reliable, I had read that in a few reviews I found online and it's encouraging to hear it from you as well
. 1981 ..... wow, that's a long time of good service!
I just wish I had a nicer case for it -- those white plastic ones just don't appeal to me, too bland, so I'll be looking for a nice old case to add character.