Ask around in your local photo forums, perhaps someone has another Zenit-B (or its successor Zenit-E) for sale/giveaway; if you intend to use your existing lens, note that they usually came with M42 mount, but some were with M39. For example, a quick search in an Estonian forum found two recent offers, for 8€ (body+lens) or 5€ (body+leather case). That's less than I'd expect materials plus time for repairing be worth. On eBay, Zenits seem to be overpriced given how widespread they once were.

When buying another one, you can still keep your original Zenit as a memento - "the old dear camera". When trying a DIY fix, you might append "... that I clumsily broke" to that. Zenit isn't exactly designed to be serviceable by user.

To me, the holes in shutter fabric tell that the shutter material has become very fragile over time, and trying to attach some fixing materials might make it much worse. But that might be just me, I've never appreciated sock darning either. If it were my camera, and I'd choose to fix/kill it instead of keeping as is, I might try using something like black rubber spray to paint over the holes. I'd try on some other fabric first to see if it gives me a thin but strong layer.