the piston will have to come back further into the end block in order to cock - this means the stroke is a bit longer, which is a good thing on a 35.
If it won't cock, I'd be suspicious that something is stopping the piston from going all the way back, e.g. the mainspring becoming coilbound (either 'cos it is now too long, or you have washers in there), or a washer / spacer on the rear guide that the piston skirt is butting up against. Final option is the spring guide itself is bottoming out on the inside face of the piston, or on a top hat (if fitted).
Easy way to test is to strip the gun, remove the spring, and shove the piston into the trigger block, making sure it cocks (it will need a firm shove, and can be released by "firing" the trigger).
Then try the same with the guuide / TH / any washers in place.
If they work, there's only the spring left to blame.
Always looking for any cheap, interesting, knackered "project" guns. Thanks, JB.