Call Sears and get the product number for the cams. If I'm not mistaken, there were two different sizes. The one my old one used was the larger of the two. The top of the machine was hinged, and inside the smaller normal cam was set . You lifted it up (or teist and lifted). New one sets in the same pins, then checked to make sure it was seated properly, and adjust stitch length to very low.
Chances are, if you lift the top, you may find part numbers, but it would be faster just to call them. My machine had a big box of them, probably fifty, with a snap closure. If you have a freecycle or craigslist, you may be able to get some there. I imagine sewing machine repair shops would charge a lot, but you never know.
Since she is a vintage machine, you may want to set up an eBay alert to let you know what comes up for that one.