[Jeremy Garchow]"What's the best way to distort the incoming screen replacement to recreate the fisheye bend?"
First, remove the fisheye with Optics Distortion (check "Reverse lens distortion"), then track and comp, then re-apply the fisheye with Optics Distortion (duplicate your prior Optics Disortion effect and uncheck "Reverse lens distortion.").
Check this out:
I don't use Mocha very often at all, but if you by chance have Mocha Pro, you can use the Lens Module.