Still plodding away on this...
The engine is back together with home made gaskets and (cover your ears) a smear of gasket goo.
I enquired about a head sealing ring, Cox and Turner had a slightly smaller on in stock but I decided to have a bash at making one. I got some 1.5mm thick copper, cut the centre out with a 50mm cutter in the bench drill and then cut the outside out with snips. After a session with the bench grinder it didn't look too bad.
I temporarily refitted the engine on the frame to make tightening the head bolts easier.
Carb stripped and given a good soak in brake cleaner.
The air filter/choke has had a bash, it was jammed open and is missing the lever. I'm going to try to split it and repair it.