Any chance that it can simply leaded up? Maybe a really thorough cleaning.
X2
No telling how it's been treated over the years with different types of ammo.Give it a through cleaning with a better lead solvent.
The only other thing you might try is replacing the firing pin spring. It could be getting inconsistent strikes but it should still shoot way better than what you are getting.
I've owned them and at 50 yards with good ammo that rifle should be capable of a 50X shot group. That's 5 in the X that's the diameter of a 22 caliber bullet.
You might also want to check out
www.rimfirecentral.com 
