@Loner, thanks for checking. I may be asking a real noobie sounding question. Will the rubber be affected by the gas/oil mix degrade the rubber at all? If not, it would be a piece of cake to cut one out of my 40 year old rear wheel inner tube when I change the tire...

Should be fine, more of an air seal rather than being constantly submersed in fuel. The seal on the fuel cap is rubber.

Some inner tubes I've tried to use for gasket material turn to a gooey mess when exposed to petroleum products for a while...  it would be okay for a while.

Butyl rubber (what many tubes are made of?) is rated "unsatisfactory" for exposure to gas and oil...  if it stays dry, fine...  but usually cap gets wet on my bikes.  YMMV.

I had a really old inner tube (early 1960's is my guess) that worked fine...  I think the rubber was different than more modern inner tubes? 

I my opinion--the washer in the cap doesn't do anything except make the cap less likely to unscrew by itself--on a vibrating old 360 lol. Just gives a slightly rubbery feel to tighten fully, although they're more plastic i think.

My original gas cap gasket for the plastic screw on gas cap (body cap Yamaha curiously calls it) still had some flex to it still, but the screw on cap itself had shrunk so much the threads would no longer engage. It was a bit of a stretch to get the old gasket over the new cap, but it made with out damage.

It looks like to be on the sure side to get a new gasket for the carb cap off ebay. I will check for the gas cap gasket too since the gasket fits the cap so snug, it has "coned" the old gasket. It is even the detent for the gasket, but....

Go to Napa and get some gasket material and cut one to size

@Ht1,Thanks for the idea, I don't know why the tightwad in me didn't think of it.....
A little off topic, your username reminds me of WWE or actually WWF with HBK and the 1-2-3 Kid

LOL Hackman101 that’s a good one I’m to skinny for WWE of course Ht1 was my first enduro rode that bike to death

I checked my stuff. Fresh out of them.

I found two old gaskets, both pretty crispy. I have a new carb and that one is fine. Very flexible. But keeping the new one. 

Niche cycle is a Mikuni dealer so should be OEM quality. Or make your own as suggested.