It is very rare to find a 4/4 Series II with an original Ford 100E engine . Most were replaced by more powerful later Ford units (usually 1500cc )

The Aquaplane head will of course help, but it is still a heavy old lump of metal and the exhaust valves are still going to be a limiting factor.

The F Type 3 Wheeler crowd know a lot about this engine and many race them . In in the USA try Fred Sissons in Indiana as he is pretty helpful.

You are totally correct about the rest of the car though , it is incredible how lack of use wrecks a perfectly good car . I know of a +4 that was raced in the 60 against Chris Lawrence that the owner parked in a garage , died , his son inherited and has done nothing with it for 20+ years. Moving it now would be a serious proposition!
That is also why my +8 has 75,000 miles on it, folks tell me that is far too many many but for a 1999 car it is barely more than 3500 pa which is just enough to keep it well maintained and used.

Andy G
1999 +8 , Indigo Blue.
Ex-John McKecknie/Mike Duncan 1955 +4 racer.