Were can I find new allen head screws for my gas cap.86 Fj1200
I don't know the size(s) of the screws you're seeking, but here's a possible source: Fastenal (https://www.fastenal.com/products/details/MS2540006A20000)
You possibly source them as easily from a home improvement or hardware store too. Take your old ones with you to match them up.
True Value Ace Hardware has a very complete selection of metric stainless steel allen heads.