Hi,
I had that same problem with both pistons on a HS 175. Clearly so had someone else - breaking the front skirt off the left hand cylinder in their attempt to resolve the problem.
I tried many of the freeing agents, leaving them down the bore for days at a time - no joy.
I tried to heat shock them with a heat gun and then ice - no joy.
They were both stuck at points in each cylinder where I could raise and lower the cylinder with the piston still stuck inside.
Eventually I purchased some M7 threaded rod with some of those longer nuts, some nice thick washers and a some sturdy steel plate.
I drilled holes in the plate that matched the stud pattern, removed the studs, then put the threaded rod through all four stud holes on the cylinder.
I put the washers and nuts on the bottom of the cylinder put a piece of wooden broom handle down the cylinder so that it stuck out the top about 2 inches then the steel plate on top and then more washers and then the nuts at the top on the cylinder to force the piston down without putting pressure on any other part of then engine.
I then cranked each nut down as if it was a head bolt / nut - opposites in turn. I did this very gradually until there was a fairly disturbing crack - this was the seal being 'broken'.
After that I just did more of the same (using longer lengths of wooden broom handle) until the piston eventually popped out the bottom of the cylinder.
Neither the piston or the bore was damaged in the process - well any more than was already done presumably the last time the engine ran.
I then got new rings and sent the cylinders to Langcourts for a going over, they matched the bores to the pistons (now cleaned up by me), Nikasiled them and rebuilt the broken skirt.
