Well, been a while, and the engine naturally arrived several weeks ago.
However, the piston (there is just one, doubleacting oscillating piston) was really stuck...as in REALLY STUCK.
Had quite a few nearby´s of de-soldering the entire cylinder-setup, and drilling the leadplugged cylinderends, to be able to bang the piston out of there.
Fortunately i kept to the penetrating oils, and just a few days ago i sensed a slight move. A few taps with a small hammer and a screwdriver, and the piston was relatively free.
The all i had to do, was spend 15 minutes gnawing out a simple half-finished flywheel, and throw a standard Mamod burner under the boiler....and it was back in running condition.
Not a master machinist by far, but nonetheless i entirely blame the crooked crankshaft for the insane flywheel-wobble.
The way too heavy flywheel was intentional. Idea being that it would help it overcome the tight spots, and wear in properly again on its own, which it sort of did.