Nice find.
With stuck opposed twins, penetrating oil, heat and time are your friends.
I would strongly advise against using a breaker bar and torque on the flywheel nut. That very possibly will lead to a broken crankshaft.
What I would do is take the power head off and immerse it in PB Blaster or the penetrant of your choosing.
If you can get the cylinders off far enough you may be able to get the rod bolts off which will make piston removal easier. If the cylinders are not bad off you may even be able to take it for a ride.
Opposed twins fire at the same time.
