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As nobody else has had a go I’ll put my twopenn’orth in. You must understand that I don’t have the deep knowledge of the minutæ of Scott variations that many will have, what I do have is the experience of making a living in the engineering game. That said I’m guessing that this engine is a blind head, (non-detachable) type? I’ve looked at some pictures of the period but cannot tell for sure.
Presumably the problem is corrosion in the bores and probably some chemical reaction between the aluminium piston and the iron block. The trouble with oxides is that they are larger in volume than the original metal they replace and chemical reactions can effectively unite the surfaces. They are also gritty all of which leads to your problem.
Attempting to use hydraulic pressure, grease/oil in the combustion space runs the risk of putting undue strain on the connecting rod and/or crankpin so if this were to be attempted I would be inclined to remove the big end rollers at least until some movement is noted. Removing the rollers also has the advantage that it separates the cylinders to the extent of two roller diameters, which immediately halves the force needed to get something to move.
Heat and cold are useful allies, cold is in some ways easier to direct. To that end can you find a source of liquid nitrogen? A lot of university departments use it. It is used in a lot of processes, there are companies that use the stuff too but finding them is another matter. If you can source some then it can be poured into the piston through the crankcase doors. That will shrink the pistons like it is going out of style and get things moving with a bit of your hydraulics. “Cardice” solid CO2 is the next best thing and probably easier to obtain, a bit slower in action so there is more time for the block to cool too but the operation is the same.
Heating the whole block might be worth a try as a last resort; quietly smoking is about as far as you should go, say about 300ºC any more will create a serious risk distortion. As it cools there is a reasonable chance that a thin oil will creep down the sides of the piston and the differential expansion rates will help break the mechanical/chemical “lock”.
Another possibility but one that has its risks is that with the big-end rollers out would it not be possible to lift the block far enough to get some steel plate in the gap between the block and the crankcase? If it is then you have the basis of a puller with some studs and more substantial section steel with provision to use the spark plug holes as an attachment point. The danger is that you are putting a strain on the big-end pin that was not intended for it.
Of one thing I am sure and that is you are surely not the first with this problem and hopefully someone will be along with a proven method.
All I can add is “The Best of British Luck!”