I have seen plastic hats for replacing the rusted out one, and these are just glued in place with silicon sealer. The repair kit I had involved cutting out the old one from underneath and glue the new one up from underneath. It had a large square collar plate around the base, probably 2 inches all the way round, and you trimmed it to suit your cut out. My cowling was way passed repairing so I opted for a total replacement, upper and lower.
One thing does puzzle me is, what stops flying insects from being sucked down through the fresh air inlet hat, chopped up by the fan and blown out through the vents into your face. Was there at one time a fabric filter that was placed over the top of the fresh air inlet hat to stop this from happening? I was debating whether to stretch a pair of the wife's stockings over the top of the hat so it acts like a filter. It is accessible to do this via the two removable plastic grills covers.