fuel cap gasket on briggs 122m02-2114

I am missing the fuel cap gasket on my Briggs and Stratton 122m02-2114 engine that came installed on my KV600SP Billy Goat Leaf Vac.  It is a flat o ring washer.  It is not a tubular ring that would fit into a notch.  I think it fell off when I was refueling.  It creates a seal between the quarter turn gas cap and the plastic gas tank.  When I look for gas cap replacements I do not see the gasket included in the pictures online.  I tried to close up the cap temporarily with some gasket material, but it does not create a good seal on the cap.  When I look at the parts breakout for the billy goat KV600SP it shows the entire engine as one unit.  When I look at the parts breakout for the engine I do not see an identifier for the gas cap and fuel tank area.  I think I am looking for the gasket that would be between the cap 799585 and the tank in a 590490, but none of the pictures that accompany either item or their descriptions indicate that there is a rubber o ring gasket that is part of the assembly.  Please help me find the right rubber o ring gasket.  Thanks.

In my original post I mistakenly used the terms washer and o ring interchangeably.  There is a tan o ring that remains on the gas cap and this appears on the replacement part 799585.  This is not what I am missing.  The part I am looking for is a black flat washer that goes between the cap and the filling hole in the gas tank.  I am guessing it was made out of viton.  It might need to be between 1/16" and 1/8" thick.  I may buy a sheet of viton and try to make one of these washers.  I wish there was an official part cut to the right dimensions.