[nota]

240 petrol motor is it? I guess you've already tried Wagga Tractor & Truck?

Good chance these guys can help you out
Automotive Surplus Online

Hillman Spares & Repairs in Melb will definately be worth a call - they used to carry a gobsmacking range of obscure vintage stuff, or probably direct you towards other vintage suppliers like Auto Spares Galore

This mob has some interesting bits & pieces
About Time Spares - International - new old stock spare parts
(btw if they've been sitting on the shelf too long then I'd be wary of using NOS rubber brake seals & hoses)

I'd be very surprised if you can't source a dizzy cap for your long-running engine series from the above list!

Failing that, have you tried other big rural truck dealers, for example long-established outfits like Intermotors near me, or maybe Gilbert & Roach etc?