Thanks everyone for the responses. I think I'll start at a LBS - having the exact specs to reference is very helpful (I combed through a lot of PDFs of old Schwinn catalogs but could not find details). Hopefully it won't get too costly. I agree that it would be best to replace all the spokes, or at least have the wheel trued/tensioned, but we'll see what's the most reasonable. Thanks to Velognome for the kind offer of spoke harvesting - I'll let you know if I get completely stuck. Until now, we've done all the maintenance/upgrades ourselves but sometimes it's best to ask the experts...

We'd like to have many more long rides on this thing but we also understand its limitations!