If using on open sights for targets, Rob, the pursuit of higher power may not be so important maybe?

But many of the older leather seal guns can make "full power" anyway.

You could go in small, cheaper, steps, and see how you go......For example, the old seal may well still be good.....just freshen it up with some neatsfoot oil. If that seal is shot, you could make a seal cheaply or, as before, obtain one from Protek. If you found that it's not making power, there's a chance that you could have the "leaky breech" issue, where voids in the braze holding the end plug into the cylinder create excess loss volume. There have been threads on how to remedy this. If you then wanted to chase power, the later piston will give extra swept volume. Power might be okay but sleeving that transfer port down will help both the power and the firing cycle feel with the synthetic seal.

I have a cosmetically challenged pre-safety 35 amongst my collection. A right old sorry state it looks in. I've kept the externals as they were and I asked Steve Pope (V-Mach) for a weak spring and his guide set. Shoots absolutely lovely and is just the ticket for shorter range open aight target use. Lovely firing cycle. It's only doing about 7.5ft.lbs and I really don't know if that's down to the spring or if I have a leaky breech, but I care not as I have lots of other "full power" rifles, including a further four 35s.

Have fun tinkering with this beautiful classic.