Yes, get them pics uploaded and posted, I may have a cap for it.
Another good place to try and find one is your local metal recycling center. The cap that Gasman linked to looks like an old snowblower or rototiller cap, like with a Briggs & Stratton motor. That's what my cap is. There were the old metal ones and
newer plastic ones too but I don't know if the the caps interchange.
Ever notice, a plastic tank usually has a metal cap, and a metal tank has the opposite?