Well after speaking with Dave... he has a few checks to do way before actually stripping the engine.
The engine is running... roughly but runnin. And does not burn any oil... I see unlikely this happening with a damaged piston.
Its been diagnosed with an endoscope... and what the guy saw trough it could be just dirt deposits from a rich combustion... or, who knows if marks from a dodgy valve... rather than the actual piston being the problem.
First step, is to check compression in all 4 cilinders and see if any difference. If compression is good and there is not oil burnt, i would be very reluctant on stripping the engine, and I would start with the ancillaries (basically injection and or spark). This is way faster to check (and needs checking regardless) than just stripping an engine for the sake of it.
Chris. Not sure what the head has in his case... normally for a mildly tuned zetec ports are already too big from factory, and i would be surprised if has any important work on it other than 3d valves cut, and despite being important... maybe not too much for the mapping.