Ooh, no idea but someone will surely be along soon.
However you will certainly be able to sort out the essentials spares-wise without too much difficulty. Piston and breech seals would be made of leather and are easily re-made.
Sourcing a spring that works won't be difficult either.
Re the barrel angle - that's almost certainly a result of a flattened breech seal. In these types of guns i.e. vintage and entry level the barrel angle is determined by the thickness of the breech seal, the breech just shuts directly onto the seal.
I would fit a new seal, then use appropriate washers behind the seal to fine tune the barrel angle, as it will be close to impossible to make up a seal that gives the right angle straight from the off - in any case it may change over time as the seal takes a set.
No need for the angle to be perfect as the sights are on the barrel, however an upwards tilt is no good as it won't allow you to use the rear sight, a little downwards will be fine.
**WANTED**: WEBLEY PATRIOT MUZZLE END; Any Diana/Original mod.50 parts, especially OPEN SIGHTS