100% stock.
I've seen similar fillers on other makes/models, but you can find this particular one on any 1997-2004 Suzuki VZ800 Marauder. Probably most Suzuki cruisers come to think of it.
The only catches I can think of... every motorcycle I've seen with that style of filler has the cap keyed to the bike's ignition. Probably fixable via locksmith, or disassembling the lock and jamming the tumblers "open". The other is the engraved "Suzuki" logo.
There's a motorcycle salvage yard a little bit west of Perth,
Woody's Cycles. They have limited daytime hours, but if you call ahead and explain what you want to do they'd probably let you walk the yard yourself.
There's another yard in Carp IIRC... the last time I looked it was in the Yellow Pages under "motorcycle wreckers".