On the transom area with the cracks. Me I would clamp a 2X4's across the top inside and out. Take a level to it and see if that straightens it out. If so lay a bunch of layers of cloth from the bottom up to that point. Let it cure. Once that is done take the braces off it should hold shape then. Then thicken that point up to the top the same way. If it is still trying to flex out just lower the braces over the new glassed area while you do the tops.
Then once that is accomplished and it holds its shape. Grind the crack out and taper it on the outside lay cloth there. Then do the pretty work on the outside.
Kinda along the lines of Chevs Idea. I would place a sheet of ply on the outside, screw it on, then do the glass on the inside. Remove ply and clean up all the hole after the new transom is in. Keep the screws away from where you need to do the prep and glass work.
Good point cause it will get through cause those cracks go all the way through, the engine was close to falling off as the rear is broken all the way through at those 2 areas, ,lots of work here on my first boat
