that's a much much more involved job than a flat transom.. fair amount of surgery there... and glasswork... plus you need to carry your glasswork/finish over into the well, etc..
after pulling the motor, you need to access the inside in the back thru whatever hatches/openings are available and check the extent of the rot on that main transom, and the construction jointing where the transom extends out the back...
in reality, the correct repair is to replace the whole transom plane of the motor, from hullside to hullside.. than retab the rear extensions to it where needed..
when you can get a chance to investigate more, take any pics you can of the inside, and I can explain what tabs to cut, etc... I hope it's got a couple reasonable access's in the back, doing that all externally is gonna be tough...