Converting to a c85 is going from a ac to a dc don’t need a dorsal fin but going to a c90 is a ccm and a dorsal fin is required. I think if you use a Buzz Wagner STC for a o-200 and 0-235 you make the rudder taller but I’m not 100% sure on that
You can go from 7AC 65HP to 7BCM 85HP (I'm doing that right now) using Aeronca Note 13 or to a 7DC using note 14. Note 14 requires bolt on dorsal fin and a couple of tubes in the fuselage. Both require Hanlon Wilson mufflers and a gas tank with a lower sump in it. I was lucky. I have the sump and muffs already. Its all theree on line. If not PM me and I'll send it to you. I did not want to bolt on that dorky fin.