Fitted a slightly larger section Viton O Ring to improve compression. It was a bit too tight to start with, but soon wore down to a nice fit. I started to have problems with starting and when I tried the spark plug outside the engine I was getting a good strong spark at the plug from the trembler coil at low revs, but completely losing the spark once speed increased. All electrical contacts appeared to be good, so came to the conclusion that the coil must be breaking down.
I purchased and fitted a Minimag MCL-1 electronic ignition kit and got a spark at all speeds, but still very weak. After further investigation, I found that the spring brass strip which makes contact with the rotating disk on the crankshaft, was vibrating and only making the lightest of contact, insufficient to give a good spark at the plug. With a bit of bending to give a tighter contact, the engine burst into life, so I now suspect the the Model T Ford trembler coil was not at fault after all. At least it will come in useful on some future engine.