Sounds like it's time for you to buy a new pump, or replacement back plate.

The back plates with vent nipples are all metal, none of the plastic ones have a vent fitting. New ones can be had from Horizon Hobbies.

You could pull the pump and make a closure plate, but I'd advise against it. The back plate also supports the carb, holding the intake pipe into the cylinder head. With the webbing cut out to allow fitting the pump it might be too weak to hold the carb correctly.

Bill.