FWIW - After a bit of experimenting I was able to get a nice clean cut on some rubberized cork gasket material both 1/16" and 1/8" thick. I used double sided tape to hold it down. I thought I’d share my settings in case anyone else comes across a need for a hard to find replacement gasket.
1/8" 2-flute straight cut bit, HSS
max RPM on my Dewalt 611 router, 27000 RPM
800 mm/m feed
Conventional cut, not climb cut
1 pass, full depth
The CAD design for the gasket was made by importing a photo of the housing flange into sketch in Onshape. The image was scaled to match some key dimensions of the original part and then a new sketch was overlaid on top of the photo. A fairly simple method of reverse engineering. The g-code was created using the Kiri:moto plugin in Onshape.