shim brass or tin, wire wheel the gasket top, use acid flux (zinc chloride) paint it over the edge of the shim stock and the gasket so it fits neat.
Tack in place with soldering iron, then solder in place along entire edge.
It won't be the prettiest fix, but if you could obtain some 0.6 or 0.8 bright tin / zinc plate from a sheetmetal guy, then you will get a pretty good result. You theoretically could make a new one out of 0.6mm and a bit of grey threebond, if you had time and patience and a good punch set.
Using a gas soldering iron (primus / sievert torch) and 70/30 sticks (plumbers solder) the job will take you max 10 min - assuming of course that you know how to use the big copper wedge irons. Once the copper block is hot enough, it won't take very much to run the entire edge seam and the job's done.
Part of me reckons that you'd probably find a valley pan gasket before you found a replacement piece of shim / sheetmetal stock to repair it...
World we live in.... can never find what you want when you need it and when you don't the bloody stuff is everywhere and worthless.

