I wasn't trying to worry you. There are options besides unscrewing that screw, but they do involve ruining a part or two. I don't have a 15-91, but I just glanced at a treadle head. If one is patient with a file, one could file down the other end of the screw that makes contact with two of the three ears.[ATTACH=CONFIG]577250[/ATTACH]
Not easy to get to and you will have steel powder residue, but it would be possible. Unless your potted motor is really in the way. try the gentle ways first.
You'll get it running.
Leonf