Another way I thought of is cut or grind away the cup exposing the rounded nut. Then a big pipe wrench. I presume the cup from a125 or 160 will fit as a replacement.
The workshop manual says that this nut is just the standard Victa 13/16" AF shouldered hex starter cup nut. No idea what previous tenants might have done, to get it to its current state!
The drum/cup is toast anyway, so I'd just take a hacksaw to it, to cut it off just above the flat section at the bottom. Grinders don't work too well, on soft metals. That way the nut's accessible for use of Vise-Grip pliers, or a Stillson/Footprint pipe wrench.
Or even, in the worst case scenario, to arc weld a length of pipe with a cross-hole drilled in it to the nut, to get it undone.
The starter drum part number is ST12353A, so it's the same as the Zip Starter cup used on the other full crank engines. And the nut is P/No ST12228A.
