I recently had to do mine on an "as is/where is" basis. Steve is correct, as usual. Although I did it from the front [on our boats you slide the steps up, then pull the bottom toward you and down], if I had a jig saw I would have cut an access panel in the back. There are screws to get the whole panel off, but I had already secured the new wiring to the sides and couldn't bring myself to remove the whole thing. Steve is also right about the exhaust hose getting corroded onto the elbow. I would soak with a corrosion breaker rather than use screw drivers to pry it off because you will stretch the hose and have trouble making it seal on reassembly.
Working from the front, I had no choice but to unbolt the assembly from the head rather than trying to unscrew the elbow alone. I would probably do that even if I were working from the rear. Before separating the elbow from the part that connects to the head, mark the angle. To disassemble the elbow, I had to resort to borrowing a big vise and an even bigger monkey wrench. Same to put it back together, getting things tight and at the proper angle. Attaching both pieces and the gasket to the head working from the front by feel alone could not have been accomplished without employing all the curse words in my vocabulary strung together in every possible permutation.
