sourcing a second hand one is an option ? i think i have one or two here but postage would make it tough huh . or change the blade boss and try a rover plate . never seen one worn out that bad , those are a cheapy so normaly chucked out when they stop starting !
must work in a sandy place huh . but same shafts as any other briggs though , just remember to check blade hight undr the deck once its fitted . too low and it is ileagle

too hight and it wont cut well . rest of the machine looks in good nick though .
those two plastic type washers fitted provide enough slip so it dont break the keyway and tight enough not to slip in use ...... no spline req'