G'day Widget,

The Briggs washer is a fibre composite material IIRC. So a suitable red fibre washer from the hardware shop plumbing supplies section would work, as would an aluminium or soft [annealed] copper washer.

It's actually cheaper to buy a complete aftermarket carby gasket kit #498261, than the genuine washer on its own. That's the way I'd go.
E.g.
https://www.outdoorking-forum.com.au/engines.../briggs-stratton-quantum-carburettor-kit


Cheers,
Gadge

"ODK Mods can explain it to you, but they can't understand it for you..."

"Crazy can be medicated, ignorance can be educated - but there is no cure for stupid..."