I'd just get it at a dealer. Boats.net was a good place to get parts online, but they seem to have stopped selling bike parts. They seem to be affiliated with partzilla.com... Try there?
Clean the gasket area well, both on case and cover. Last time I did a stator, I used silicone seal (black gasket sealer) on that 3 wire grommet. Goop a bit of sealer on the wires, then gently pull the grommet over the goopy part of the wires to the proper position. This will get the sealer into the grommet to seal the wires better. Put a bead of sealer right around the grommet to seal it to the case and cover.
Don't try to reassemble that cover without a gasket. You evidently need the spacing of the gasket thickness to keep things from rubbing inside the case. Wouldn't hurt to put sealer on the gasket, both sides, to make sure.
The big magnet in the flywheel may make removing the case cover tricky. It can hold things together more than you may expect.