Regarding the Webley Rebel breeches that chambers sell, yes they are inferior quality and they will be marked as webley not Sharps, but .... at least you will be able to mount your scope and shoot your Innova. If you never intend to sell it then why not.
Regarding repair of the Innova breech, it may be possible to have it plastic welded by a specialist. Particularly a motorcycle repair specialist as they repair plastic fairing panels on a regular basis. You will have to re-cut/ file your scope grooves back in though. I don't know if this has everbeen done...?
Pete.