Hi Norm
You would need 5.5 mm from standard boss to the pulley .

The eBay ones are $70 with postage. If it fixes the problem the owner should be happy to pay that though.

https://www.ebay.com.au/itm/SLEEVE-PULLEY-for-self-propelled-Victa-Mower-EN73016B/192769908291

What about 5 mm steel plate cut a strip ,heat with the oxy and bend it around the boss and weld that's if
you have the 5mm clearance .

You may be able to weld a small round plate to the crank nut then have a bigger heavier nut welded to that.,like a tow ball
nut.

Cheers
Max.