If it's piston/ring related, (doubtful) you would have clouds of billowing smoke. I pulled the injectors and got our works cheap scope camera down into each intake chamber and also down the spark plug hole(no holes there!), piton was fine, valve edge looks burnt on 6, so it must be valve related(Compression test also 0 on Cyl 6).
See attached images most looked very clean I was surprised by how clean the intake chamber was generally, if you look you can just see the burnt valve edge showing signs of blacking up.(second image)
might be cheaper to buy on of these and see for yourself, good luck. Planning on stripping down the head next weekend. I'm also looking for options on rebuild costs.
I'll keep you posted.
https://www.amazon.co.uk/Endoscopic-...zcF9hdGY&psc=1