I would not reuse the rim, that is just my opinion though. 30+ years on a stressed rim would not lead to a long life tied to another hub.
As for the rear hub, you can carefully repack it with grease, the bearings are undersized and can fall everywhere. So be careful. That is actually the issue with the rear hub on Helico's the undersized bearings. So you can ride them just when they fail, they fail. It shouldn't be a spectacular explosion, just a lot of grinding. However if you have a lot of fast descents where you live then you may want to go ahead. At lease repacking would buy you time. As for the cassette itself, the lock ring can be removed carefully with Channellocks, it is delicate though so be nice to it. Front hub is perfectly fine.
As for a new rear wheel, any Suntour or Shimano based rear hub that accepts 7 speed cassettes or freehubs should work. You will have to get a new cassette or freehub as well though because the Mallaird isn't compatible with anything else. Swap meets or asking the gentlemen on here may be a good way to find a replacement.