Sorry to hear of your engine trouble. I hope you are well on the way to getting it sorted out. If oversize pistons are not available, could you not have that one cylinder bore sleeved back to standard?
Also, to answer your earlier question; that is not an overdrive gearbox. The overdrive is a planetary unit in a finned housing on the back of the three speed box, with the handbrake mounted on the rear of that. The overdrive is operated by centrifugal action, there is no electrical supply. However, my 35 Airflow SE has a lockout facility to prevent the freewheeling from working on long descents. That takes the form of a large knob on the dashboard that is connected by a heavy steel cable to the transmission housing. I wonder if that cut-off cable you have seen is in fact the lock-out cable?
You appear to have the automatic choke on your carb. Mine has been reduced to a manual cable somewhere in the dim and distant past. The car was acquired by my father in 1969 and has therefore been in our family for more than half its life, but the choke was changed well before that.
Did you have difficulty removing the alloy cylinder head? Mine has never been off and I hope it does not need to anytime soon, but I was concerned that it might be difficult to remove.
Adam..