I'm not sure what the exact fix is. Have you cleaned and oiled the machine? Have you tried holding the lever with one hand and turn the hand wheel with the other? If while doing that if the hand wheel doesn't turn (or is hard to turn), I think it might have some gunk somewhere that needs to be cleaned out and oiled well. It might just be a nut on the lever needs tightening. It looks like you have other machines with the same stitch length regulators (201s & 15) that you can compare this machine to.
A couple of links that you can check out
http://web.archive.org/web/201610160...achine_manual/ All the pdfs there are worth downloading and quite helpful. I think the one for the stitch length is "The Machine Top"
The other is
http://malepatternboldness.blogspot....ger-15-91.html Maybe read down through the comments.
Janey - Neat people never make the exciting discoveries I do.