Looks like a difficult shape to cut by hand. There are outfits that will make custom gaskets, CNC-cut, from a template (which could be an old gasket); best guess $10 a pop for these but I could be way off. For the dedicated purist, that should be reasonable enough.
Have to wonder if a stator plate O-ring could be adapted. It's thin, as O-rings go and compressible. A single slice would allow you to increase the diameter to match the mating surfaces. A little weatherstrip adhesive, applied to the cover, would hold it in place. And a teensy dab of RTV could then be use to seal the gap, which you'd located at the 12 o'clock position. From a purely practical standpoint, RTV could be applied to the cover, then allowed to cure before installing it. I usually cringe at the thought of gasket sealer, let alone RTV silicone, use on these motors but, this is safe enough and accessible enough to clean, if one uses too much. IMHO, you'd want a layer that's thinner than the paper gasket; that, plus pre-curing, should eliminate the old, dreaded, squish-out.