Between getting home and starting on the frame I attacked the engine. These AV7 units are so easy to work on that I had the bottom end stripped, cleaned, new bearings and seals installed, and rebuilt in a couple of hours.
The bad news was that the original cylinder was unuseable. There was a deep dink in the top of the bore, so I pulled a spare barrel from a box of bits and installed that. Yes I know it's from a Raleigh, that's why it's curved on the fins, but all the bits are interchangeable so... Oh and I used a spare cylinder head as the one that came with the bike has a seized decompressor valve.
Still after a bit of work it feels right and has good compression, not to mention looking better.