I'm not sure exactly what you're asking, so if I appear to be talking down to you, please don't take it that way - I just want to make sure that you have all the info you need.
That thumbscrew sticking out of the silver-colored plate on the front of the pillar should unscrew counter-clockwise so that you can move it up or down. Move it to where the stitches are the largest they'll go. (In that era, 6 is probably the biggest stitch you could usually get - 6 stitches per inch) To keep the stitch length from jiggling out of position, screw it down clockwise at that setting and see if the machine is now making the longer stitches. Count the number of stitches in one inch of seam. It should be 6, if you have the machine set at six.
If that didn't make any difference, or if it did make a difference, but you're only getting a little bit longer stitches, my guess is that something that's driving the feed dogs is stuck - probably with old gunky oil or something. Bathe all the moving parts inside the pillar and on the underside of the machine with TriFlow or regular sewing machine oil and run the machine at full speed until the feed dogs are loosened up and making the stitches as big as you have the machine set to make.
OR until the motor starts smoking, which leads to the second half of your question - what's wrong with the motor?
The intermittent operation could be caused by a broken foot control, a bad connection anywhere in the wiring, stuck bearings in the motor, worn brushes in the motor, etc.
That Atlas is definitely worth saving and if the paint on yours is as good as the one shown on stitchnerd, I would put the money and effort into getting it refurbished.
Replacing brushes and re-wiring foot controls is easy-peasy. Jenny at sew-classic.com is the bomb - she's got everything you need.
Do you have a manual for the machine? I think I saw one online, but if there's not a free version floating around, I'm sure Linda at Relics will have one. And there are lots of people here to help you, too.