As you say, the Guzzi 250/4 is just a re-branded Benelli as both factories were owned by Argentinian entrepreneur Alejandro Tomaso at the time.
I doubt you'd find much in Australia so I reckon you'll have to search worldwide.
Below is a list of classic Benelli parts suppliers that I found useful a few years ago when I got my Benelli 900 Sei. The list is several years old but I think they're all still trading and the contact details are still current.
Otherwise, get in touch with the Benelli/Motobi Owners Club UK. They'll have up to date info on the best sources of parts for old Benellis. They may even be able to put you onto a club member who's breaking a 250/4
Website:
https://benelliclubgb.net
Email:
info@benelliclubgb.net
There's always general markets like gumtree and eBay, but you'd have to be lucky to find the part you need fir a rare bike like that.
Benelli parts suppliers
Selwyn Motorcycles
Parts for 1973-1980s Benellis
Cambridgeshire, England. Tel: 01223 893331. Mobile: 07711 022925.
Website:
https://www.selwynmotorcycles.co.uk
Email:
SELWYNMOTORCYCLES@LIVE.COM
Detlef Burian
Germany. Parts for all Benelli models. A selection of Motobi and Wards Riverside. Tons of spares (ex Cosmopolitan). Order directly off the Website
www.benelliparts.de Tel: 49 5144 2592. Fax: 49 5144 2592.
E-mail:
burian@t-online.de
Moto Stefano
Germany. Incomplete bikes for spares. Tel: 49 7433 381486.
E-mail:
info@motostefano.de
Eric Daminet (not sure if he's still trading)
Belgium. Tel: 7171 3499
E-mail:
archives@benelliclubgb.net
For non-engine spares tel: 01823 451732, e-mail:
editor@benelliclubgb.net