There is a fast [destructive] chemical way to remove the piston, without any damage to the bore. These solutions will attack rust, but not iron/steel.
But it does require some access to the underside, so as to part fill the inverted piston with the alkaline chemical solution. 5mm would be sufficient, if a syringe and plastic tubing is used.
Inverted - so that when the piston crown is eaten through, the solution can run out into a catch tray, not into the crankcase!
The 'mild' approach is to use Trisodium Phosphate, available from hardware/paint shops as 'Tricleanium'. Say at 5% w/v; 5 grams per 100 mL of water.
BTW, half that concentration, i.e. 25 g/L, works well for cleaning range hood filters, but keep a close eye on them - it works rapidly on those.
Whereas the 'aggressive' approach is to use a Caustic Soda [Sodium Hydroxide] solution. I'd try this @ 5%, too. Add the soda pellets to cold water.
Safety: face/eye protection and rubber gloves are strongly advised, when preparing/using these chemicals.
