Hej Hans,
if you had posted a month ago, I could have come round for a look. I visited my wife's sister in Svelgen in August. Anyway, like Dan writes, pour some penetrating oil (or diesel) in the spark plug holes. Let it sit for a week, then try turning the pony flywheel back and forth. If there is zero movement, take the pony cylinder heads off and get a piece of 50x100 wood and 2kg hammer and tap on the pistons with light to medium strength. If it seems that the pistons are loose, then you might need to remove the magneto and try again. If there is a small amount of pony flywheel rotation, pull the drain plug underneath the pony clutch pinion housing and see if any water drains out. If it does, the pinion gears might be rusted up. If not, you might need to remove the top cover off the pony to take a look inside. Take photos and post them here so we can take a look and advise next steps.