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This is not at all uncommon. Often piston rings will seize from corrosion after sitting for a few decades. It almost standard procedure to to pull out the spark plugs and soak with penetrsting oil and then use a breaker bar on the crankshaft to work the engine free. 90% of time this works and you can move on. If that fails then you have to start assuming something else is locking up the engine such and a broken chain, stuck valve, seized shaft, or major mechanical damage. 

I have had luck freeing stuck motorcycle engines  using penetrating oil and rock the bike back and forth in 4th gear. Or rolling it and dropping the clutch in gear... with the plugs out of coarse.  I just had to do this a 1969 H2 triple. The engine did break loose after a few tries, I even got it to start and run after some tinkering.

Thanks for the help Kawboy, Im still trying to free the pistons and have been spraying KROIL all over the cylinders for days with the hope that in the new year the seized pistons will raise the white flag and surrender to my efforts, once free I will search out someone who can precision plateau the the cylinders, once again thanks for the info and hope you and yours have a safe and Merry Christmas.

Seized an old beater hand me down lawn mower turned weed muncher. Montgomery Wards mower, big wheel, Briggs and Stratton 4 hp. It ran great on race gas! Removed the governor, the exhaust muffler ran cherry red. 

Then she seized.

No problem, pour a lot of Kroil in there, wait a couple of days, then use a 2x2x6 chunk o' wood and smack piston crown with small sledge! It worked!

And that mower than until I sold it for 5 bucks.

Good Luck!

Well eventually have stripped the engine down, #2 con rod bearing had spun and has caused minor surface damage to the crankshaft, also #1 & #3 cylinders (the stuck ones),as predicted were rusted to the cylinder walls, but after splitting the crankcases removing the gears etc, (with much KROIL and intense encouragement) the pistons were liberated from their place of residence. My question is #1 cylinder looks quite pitted and may need a rebore so ill end up doing all of them (even though none of them are scored/pitted other than #1) what is the maximum you can bore out the block should it need to go that far?

You can go to 64mm on stock cylinder sleeves. The lower skirt gets really thin but those who have gone this route have not had any skirt breakage.

Cruzin Image sells piston kits on EBay and many have used these. The pistons are fine but the rings they supply with the pistons are shit. Probably cast iron rings. Some have used the Cruzin Image rings with no trouble. Others have broken a ring when installing the rings on the piston.

IMD also offers pistons with rings and the rings are a good quality so something to consider. You might be better off in the long run going straight to IMD  They also offer pistons in 62mm, 62.5mm and the 63mm. $547.00 CDN They also offer 62mm piston ring sets for $304 CDN but oddly enough, do not offer rings in any other size?? That would be worth an email.

So let's say you went with the Cruzin Image piston kits at $ 320 CDN and chucked the rings and ordered rings from IMD at $304 CDN. Now you're up to  $624 + shipping from 2 different locations. Further off just going straight to IMD and get everything.

IMD piston kit for carbed engines

Thanks for the IMD info, I've just got back from the engineering shop and the news is bad, crank damage is pretty bad, camshaft rust is more than surface rust so currently trying to source replacements for both cams and crankshaft, (have put an offer in on ebay for both), on the upside the cylinders can be re-bored, ah well every cloud I guess!
My best option is as planned to replace everything internally with new items, looked at the con rods on the MCG site, and would be up for €1387 ($2264) for the rods alone! are there any other options out there for rods etc? it seems to me that I've bought an absolute lemon, if I add in the cost of the replacement crank, cams, re-bore, pistons, gaskets, valves and all the other ancillaries I'll be looking at close to $10,000 on the engine alone.