Keep the Skinny Birds Flying Safely
As far as I know, Joel, Joe Rankin's Collegiate is the only one with a C-85. Back in the mid-50's, my Collegiate, a FP-65 was upgraded to a Franklin 90, but using the accessory case of the original Franklin 65 to still be able to use the existing mount (so no starter & generator). The major concerns the CAA had were: weight & balance; adequate fuel flow at climb attitude at full power; and adequate carb heat, which required rerouting the cabin heat duct from its muff to the carb airbox. The irony is that Joe could have produced enough supporting data, as the TC holder, to have the C-85 added to the list of acceptable engines on the TCDS, but he lacked the funds and the enthusiasm. I've never seen his drawings for his C-85 installation, so I have no idea how it differs from a stock CP-65. Joe's son is sorting through Joe's documentation for Porterfield (Rankin) Aircraft, but he admonished me that he and his bride are also preoccupied with running their FBO/maintenance facility/flight school, so it will take a while before Joe's field approval documentation is available for you to use as a basis for your C-85 conversion.
I can scan and send you the documentation for 41Vermont's Franklin 90 upgrade, and maybe you could use that as a basis for your upgrade, but the FAA has been hesitant to grant field approvals for the last decade or so...but maybe your MIDO is more interested in antique airplanes than the Boston MIDO, which seems terrified of them.