My 2c on the springers:

HR81 - old one has a firing cycle that sounds like a car crash. Please make sure the basics, like properly fitting guides, and sensible transfer port volumes, are addressed to avoid horrendous slam and poor power.

Service - nice, just that horrendous rearsight. Nothing wrong with making it scope-able, but 90% will want either a peepsight or regular opens.

In terms of addressing issues with the original, in order of priority I would:
- reduce the transfer port volume / length to avoid slam and increase power
- shorter barrel (already seems to have been done) - 18" is plenty
- increase the stroke - making the main cylinder a little longer if needed would be easily accodomated - say +15mm
- parachute lip type piston seal
- shim the pivot pin each side
- take the slack out of the removable barrel mounting
- trigger

Finally, if you really want to sell something that will be very popular, make a modern Bonehill britiania - it's already a much better gun the the 40-year newer Webley to start with, so should be less work to repro