Does anyone know who would rebuild this shock or if there is a way I can refill it ? Thanks 

sagesuspension.com/ or RaceTech

mountain bike high pressure shock pump and needle will work in a pinch, best to use nitrogen, any good off road shop should be able to do it, valve is near rear shock mount.  (250 psi)

The damper adjusting rod #19 corrodes into the shock shaft after 40 years of no adjustment.
It's made of Duralumin 'or something'.
If you back off the damping ring click by click & it keeps pressure on the clicker you're all good.
If it stays put & the clicker looses tension then the rod it stuck--unless it's lost its gas in which case you end up with the same effect ie no gas piston compressing the oil so no oil pressure to push the adjust needle up with the rod.
If it clicks up constant you can click it down until you run out of click, about 32 clicks. But DON'T bottom it out forcing one more or the needle can jam down on maximum damping.
Sorry, got a bit carried away writing an essay. Just reminiscing on my days in the local Yamy shop back then.
Fun times & lots to learn as things improved--or not always improved.

www.yamahapartshouse.com/oemparts/a/yam/...c7859d0/rear-cushion

Good stuff , thanks 

Had to have a look to see what i wrote haha. Cheers.