175 Hydraulic Remote Valve interchangeably

Once the spool is sliding in the sleeve, you should be able to use a large flathead to unthread #49 from the spool (make sure you don't clamp on the polished sealing surfaces of the spool when holding it from turning). You technically don't even have to do that if you slide the sleeve off the front (it just makes it harder to slide back on without cutting the o-ring inside the sleeve). If you get this far, in 'theory' you should disassemble and clean/renew all the gubbins inside the spool (the adjustment set-screw, shuttle spool, etc. numbers 39, 40, 41, 42, 43, 50, 51). But if it were mine, I wouldn't bother. That adjusting set-screw (#51) is a PIA to remove and it's a pain to deal with everything inside the spool and get it back together sealed and adjusted right. At this age, you probably need heat to be able to unthread it, and even then it's so small (and likely rusted) that it'll be hard not to strip the head trying to remove it. And if you've used heat, you've likely wrecked o-ring #50. So then you're really screwed if you've wrecked the o-ring and you can't get the set-screw out to replace it.

If yours is as rusted as it sounds like, I'm guessing all the internal shuttle components (which I believe are all essentially just to make sure the detent unlatches when pressure is built) will be corroded/pooched anyway. I'd just keep the spool with everything in it untouched, remove the detent balls, lube it up, and reassemble. You won't have detent anymore, but it'll still work dandy. Be sure to replace o-rings #44 & #48 when you reassemble. You should theoretically replace the front o-ring as well (#8), but not quite as critical - the leading edge of the spool where the tangs are has a slightly broken leading edge (not a sharp machined edge) so as long as you polish and lube the spool prior to assembly you probably haven't hurt it. I'm 95% sure there's also an o-ring inside that sleeve that's not shown in this picture. That should be replaced as well, and you have to be careful when sliding the spool through it that the sharp machined edges don't catch & cut it. If you were able to unthread #49, slide the sleeve on the back of the spool, rather than sliding the spool all the way through the sleeve. If #49 is still on the spool, just be careful when sliding the spool through the sleeve and o-ring. And use lots of lube and be gentle.