Probably the mechanic wasn't familiar or comfortable working on your machine, I'd find another mechanic if possible. If you want to try fixing yourself, I'd get a can of PB Blaster or some other similar nut loosener product, and trace the path of Reverse and stitch length all the way to the feed dogs and spray wherever two parts rub together. Should free things up if you work the controls back and forth. Most likely somewhere along the path something is gummed up preventing forward movement. Old WD40 and gummy lubricant acts as a powerful glue in the older sewing machines.
Jon