I’m not that familiar with the fasteners being discussed, but I sure don’t like the looks of that cracked cover! I know you guys value originality, but I’d probably change that scheme to the O-ring and flat washer idea. You need a 1/16" thick O-ring with an ID that fits the stud and a flat washer with an ID that matches the OD of the O-ring – meaning 1/8" larger than the stud. Set O-ring around the stud, set the flat washer around that, and tighten the domed nut just until it sits hard on the flat washer – which means it has already compressed the O-ring plenty to achieve a reliable seal. The O-ring serves as a lock washer and will prevent the domed nuts from loosening.
The OD of the flat washer doesn’t matter. So if you wanna do it right you can use copper washers with the same OD as the original washers. With any luck you won’t be able to see the O-rings and the assembly will look exactly original.
The engine just might get too hot for normal O-rings. It’d probably be OK, but if concerned you can get Viton O-rings. If you burn those up, you’ve burnt up your engine.