And any chance the old valves are useful/sellable or are they just more scrap weight?
Most older valves are worthless, but there are exceptions.
Older J-valves can be made to work but on the ones I've seen, the J mechanism is on the tank side of the valve seat and therefore always pressurized. They are a fruitful source of leaks even if you're not going to try to get the reserve mechanism to work. In many if not most cases the reserve mechanism will require service and the parts just aren't available. If you're lucky and you have one that was lightly used and in good shape, you can maybe get it to work with the addition of some O-rings (not the standard SCUBA ones, you'll be hitting McMaster-Carr or similar).
Anything with a pin gauge on it (pressure gauge using a piston and spring mechanism with a visible pin showing approximate amount of air left) is going to leak a little unless it is in fantastic shape, because even a tiny scratch in the pin is going to cause a leak. Here again service parts are problematic.
You can get really good valves at DGX (and several other places) for $39 with (usually) free freight. These are combination DIN-yoke valves. A valve parts kit and burst disc assembly, from DGX, to overhaul an existing valve costs $24. Therefore, there is an upper bound on the value of a "needs overhaul" valve of around $15 if your time and fiddling are worth nothing.
If you like to dive vintage gear, well, great, use a vintage valve, they're cool.
There are a few configurations of valves that are either no longer made, or expensive, or hard to find, and those are worth hanging onto. This would include oddball stuff like valves for triples, and valves for tapered threads or 7/8" UNF or M18 cylinder necks. I've never seen an aluminum dive cylinder with anything but the now-standard 3/4 NPSM threads, the oddball stuff was all used on steel.
