Hi Robert. What an interesting problem you have. I have been very reluctant to make a suggestion as one man’s interpretation of what constitutes excessive force can vary widely from another person.
We do not have easy access to low temperature liquids, so if we were faced with this problem, I would first be sure that I had tried all traditional methods. I would have soaked the whole thing in a bucket of diesel for about two weeks.
I am assuming that you have removed transfer port covers and have examined those areas of the pistons that you can see for evidence of corrosion.
If you have access to suitable machinery or tooling, I suggest that you make two recesses in the door seating face that are co incident with the position of the crank screws. Copy the original Scott features and be careful to leave some sealing face for the door.
Remove both screws, outer plates and rollers.
If the stiffness is due to corrosion of the pistons in the barrel, then you should be able to rotate the flywheel a little, as we need to isolate exactly where the problem is
Check condition of rollers and big end bearing surfaces, this may give some useful indication as to the extent of any corrosion
If rollers and bearing surfaces are in good condition, replace rollers on one side only and try to move piston by rotating flywheel.
Now repeat with the opposite side,
Do we have movement one side but not the other
If both pistons are solid but the big end bearing tracks are in good condition,
Turn up a dummy ring in aluminium or brass to take the place of the rollers, to avoid brinelling of the track by the rollers when extra force has to be applied
Place in one big end
Clamp the crankcase firmly to a bench
Turn an aluminium bar to pass through the spark plug hole
Get a friend to pull the flywheel in the direction that is correct for piston on the downstroke
Give the aluminium bar a hit with a medium lump hammer
Repeat the process on the other piston
Consider that if the pistons are corroded in badly, then they are hardly likely to be useable and if necessary could be considered sacrificial.
I have found that if a steady force is applied and then a blow from a medium lump hammer with reasonable mass is applied in addition, then this will often solve the problem in hand.
Best of luck and kind regards Roger Moss