I once had a Newton missing the aperature and was able to screw in the smallest Lyman ( could have been a Marble's) aperature available. It worked. An old 256 I had was missing the aperature and someone had forced in the base of a rimfire shell ( 25 RF?) and puched a small peephole in it to work as an expedient. I never bothered to improve on that. You're fortunate to find a .35 in that condition. The original sling should be not leather but a web sling.