I would imagine a radiator meant for a Mustang with, say, a 289 and air conditioning would have plenty of capacity for cooling a C169 or C175. I do know that there were 2-row and 4-row radiators for the '65 Mustang, as I have an example of each in my barn. I think there was also a 3-row variety.
I've read that hot rodders will sometimes have custom aluminum radiators made. I don't know about the cost, but it may be possible to have something fabricated that's close to the original look without getting too expensive -- just a thought.
Mark W. in MI