I think the air cooled R80 would be the best bet to use as it's just that bit more powerful, with more torque, yet is a similar size and shape to the original 2cv unit so may be able to retain the cowl set up for cooling. It must have been done already somewhere!

But where would you attach the fan on an Airhead engine? If we are talking about fitting a BMW bike engine to a Lomax, there would be no need for a fan and 2CV engine ducting. Both engines are good but of course the newer Oilhead engines at 1100/1150/1200cc sizes have lots more power, probably more than a Lomax needs.