What type of screens do I need for this crank case window?
The screen will sit where the blue lines are, up against that small lip (sorry bout the poor drawing skills). It will be held in by flush mount clips like shown below. You will need to drill small pilot holes for the clip screws.

You can have the screens built to your custom size at a good hardware store or home center. Do not get the cheap screens with the lame plastic corner pieces, get good heavy gauge mitered aluminum frames.