Christo, I don't think that JulianB's mower would have had that cover, due to its cowl-mounted throttle control.
Every old Masport I've seen with that throttle setup, just uses the engine cowl as the top cover.
In anticipation of finding I have a warped casing and need a washer, where do I source such an insanely small part?
Errggh. That's the curly question. The 391413 repair kits are readily available in the US, but generally only under B&S P/No 4184, which is a 'workshop pack' of 10 sets. Not that dear [can be got for <US$8], but postage will cost more than the parts! jackssmallengines.com is one US parts mob who will accept international orders, but it's necessary to go right through their checkout process to see the postage cost.
Have you redone that flatness check, with the feeler gauge, as in the above diagram? That will tell us if the washer is actually required here, although it looks like it is.
If you're accustomed to using hand tools, grumpy's method of draw filing the tank top back to flat is worth considering. We can find an online technique how-to, if needed, I'm sure.
All you need is a way of holding the tank; something like a 'B&D Workmate' [or clone] portable bench/vise setup works quite well, if you don't have a decent sized bench vise [with soft jaw covers]; and a good sized [250-300mm] fairly fine
single cut flat file. And a wire brush, to clean the file teeth.
