1980 Yamaha IT 125G rear shock question

Hello, I am new to this forum, but not new to forums.I was given the above bike and I have gotten it running and I enjoy riding it as often as I can. My dilemna however is that the rear shock is sagging, a lot, and I cannot find where it can be recharged with more nitrogen.
Would a new spring bring it up to where it should be? I have the adjusting collar all the way up right now, but it is nearly bottomed out when I sit on it. True, I am probably too big for this bike, as I am six foot plus and 240lbs.
Would a shock from another year/size bike fit on here? Am I doomed to having a bike with a saggy ass?
Any help is greatly appreciated!

Thanks in advance

Bump. Does anyone know of any shocks from other models that would work?

Yeah I am thinking that this doesn't have many active participants. Everyone mus t be into Modern $7000+ bikes!

Thanks for looking. I have found Noleen J6 and Pro Action Worldwide as two spots that rebuild the rear shocks.