Boat Motor Switch 90hp Yamaha for a 50hp Merc - Advice

I would love to get some advice from the pros out there. I have a pontoon that has a 50 hp merc on it, and a smaller monarch bass boat with a 90hp yamaha on it. The pontoon is the primary fishing device due to mama and chasing stripers. Make a long story short, I want to switch the motors. My strategy is to label everything, unhook from under the dash panels, switch the motors and go from there. I already know I will have some issues with controller mounting locations, and cable lengths. Just wondering if there are any decent guides or websites to step you through it. Some of the items such as the oil level light (red, yellow, green) I am really wondering how to setup on the pontoon. Do I take the whole guage? Things like tach, trim indicator, water pressure, are most of these universal, or am I in big trouble. The 50hp merc has a hole in the #2 piston due to oil clogging up only in that carb. I have been estimated $2500-$3500 for the repair. I am going to switch the motors and then save up some money to fix the merc and get the basss boat back in action. Any advice or help is GREATLY appreciated!!