:welcome: to the site, Little Dipper!
That is one very cool and unique machine that you've inherited! I love stories like this, it makes the machine and experience all that more meaningful. Keep it in your family and don't ever get rid of it.
Bruce provided some great tips on what to do and check for to get it running again. Regarding belts, I would take them into a lawnmower shop and see how far you get with them in terms of having them ordered or maybe just maybe, they might have one or two in stock of the right size. I would also try and locate any part #'s that might still be on the original belts if they are legible.
It is really neat that it has the sulky to go along with it as well. Definitely keep us posted and if it at all possible, a video would be awesome! :cool