OK ...
Here's how I'd do it.Take the bolt pattern of your OB mount and find the area of the mount base at the transom.Make two motorboard pads out of 3/4" ply. One twice the area of the mount, and the second one 4 times the area of the mount.The large one goes inside the transom and the smaller one on top of the first one.Sand/grind the inside of the transom to clean the surface and expose the fibre-glass. Shape the first doubler to fit the curve of the transom, doesn't have to be super close, 1/8" gaps are ok.Get some heavy FG mat to act as a pad between the transom and the first doubler (the mat will fill any gaps). Drill the holes for the motor mount through the transom and the two motor boards so the mount is centred on the doubler's. Do a couple of dry runs to make sure it all fits. Make sure the mounting bolts are long enough to go through the transom, both boards and leave an inch or so for washers and nuts.When you are happy with the fit, mix up a batch of epoxy. Coat the inside of the transom and both sides and all edges of both motor boards. Let the epoxy set for 12-24 hours.Cut the mat to fit the large board, two or three layers would be good. Use some temporary bolts (spray the bolts with PAM so the epoxy won't stick to them) to locate holes in the mat and wet the mat out with epoxy, one layer at a time. Take the large board with the mat and install it inside the transom while the epoxy is still liquid. Use the bolts to clamp the board to the transom (a bunch of epoxy should ooze out from the mat).After the epoxy sets, remove the bolts. Coat the back of the large motor board with another coat of epoxy and glue the small board on. If the small board doesn't fit flat on the first board add some mat between the two. Respray the bolts with PAM and use them for clamps.After the epoxy sets, you have all the load from the motor spread out over a large area of the transom and a solid laminate that shouldn't stress the transom/hull joint.To finish it off, I'd add thickened epoxy fillets all around both boards and 2-3 layers of medium to heavy FG cloth over the whole thing. Run the first layer about 3" past the edge of the boards, and 3" more on each layer after.The idea is to prevent any point loads that could crack the transom.Catalina's are built pretty heavy, I'd guess the transom is about 1/4" thick if not more. You might be able to use 1/2" ply instead of 3/4".No need to use marine grade plywood, since the mount won't be underwater and all the wood will be sealed in epoxy. Exterior A-B should be fine.As a point of reference, the transom of the runabout I'm building has a 1 1/2" (2 layers of 3/4") thick motorboard and it has a 15HP motor and weighs about 200#. The loads on the transom of your C-27 will be higher when the motor is tilted up and the boat goes through waves.I'm glad to help, I just wanted to point out some of the trade-offs when you replace an inboard with outboard power.