Okay, it sounds like the machine was previously used for leather and had some upholstery weight thread and a leather needle. So I was thinking stick with what the machine wants to do and get more of the same type thread. Its true and quite frustrating that a lot of companies don't specify their thread weight, hence the suggestion of Coats and Clarks upholstery thread which looks and feels pretty heavy duty to me, but no I don't know what weight it is. I have to admit I don't know where else you could get it, maybe search online?
Alternatively, do you have a way to change out the needle or other needles that might have come with the machine? That would help with using your Mom's old thread which is very likely 50 weight. I'll usually use a 80/12 or 90/14 size needle with 50 weight thread, and the 90/14 would be best for cotton duck fabric. Sigh, and the needle size is embossed on the side of the needle, which needs a magnifying glass, good light, and patience to decipher. Another issue is whether your machine will accept the currently manufactured needles. A few years back I bought a Singer treadle machine in a moment of total insanity and the needle on it is huge. Doubt if anyone even makes them. And of course its rusted. But I digress.
Here's a link to Schmetz needles with some info:
Hope something here helps you out. Then there's always glue....