To remove the boss, lay it on a hard, solid surface [an anvil is ideal, or the side of a club hammer head will do], and give the upper side a smack or two with a steel hammer. That will pop it off the shaft taper.
We really need some pics of the engine, cowling and starter to date the mower accurately, but it looks like a mid-late 1970's Ute, as it has the modified crankcase half for the bottom seal protector disc.
Have a look through the Victa catalogues of that era
HERE for an idea of colours - the Utes stayed with red base and cowling and cream handles for the later 70 Series mowers, once the Zip Start system came in, in 1972.
