+1 to bigted re the boss [pullers rarely work on these, but impact always does], and Gizmo re the top nut [it's a normal right hand thread BTW].
I'd add that the top nut is usually the first thing to remove in a teardown of a Victa full crankshaft engine, preferably while the engine is still mounted to the chassis [better leverage].
If you want to get it undone with hand tools, you'll really have to reverse a couple of steps in your disassembly. Those being, run the nuts back up along the crankcase bolts [don't need to be super tight though], and reinstall the cylinder and head [don't need the gaskets]. Then you can pack the combustion chamber with rope/cord [the softer rope the better; cotton is ideal] fed through the spark plug hole, to lock the crank. This permits gripping the crankcase in a vise [not done up tight; just enough to engage the corner lugs fully is fine], and getting a breaker bar and socket on to the top nut to undo it.
From the description of the initial problem in the Whirlpool thread, I'd say that all that really needed to be done was to tighten up/replace the blade disc nut; the blade disc is just held tight by friction between the washer and boss; the boss lugs and the slots in the washer lock these two parts together.
There's a bit of a trick to getting that nut tight enough though; a rattle gun is the go. Or in the days before rattle guns were common, we used to use a big ring spanner, and hit it with a hammer for the final tighten, while hanging on to the the edge of the blade disc. My 1" AF spanner still shows hammer marks...
The nylon locking insert in the blade disc nut is obviously cactus, which is why it worked loose.
However, having gone this far, you might as well do the seals, and check the crankshaft bearings etc.
