When I went through this exercise I concluded that the corvair was just too heavy, based on the weights I was able to find. Here's the engines I considered:
...............Ramp weight...Lost payload...HP
Lycoming O-23........250......87................118
Corvair O-164.........225......62................100
Continental O-200...215......52................100
Subaru E81............200......37................100
Jabiru 3300cc.........178......15................120
Aerovee VW...........166......3...................80
Rotax 912..............163......0...................80
Jabiru 2200cc.........132.....-31.................85
I got real excited about the Aerovee but after some research I became skeptical about the HP claims. The company wasn't interested in helping with a Kitfox installation, either. Great Plains VW might be an option, though.
Then a used Jab 2200 turned up and ended the debate.
Besides the lost payload I was worried how far forward an extra 60 pounds up front would move the CG. I once considered buying a Model 5 locally that had a Subie up front. It had a huge hunk of lead in the tail to make it balance.