Buy the motor your talking about. There will be no gain to a 55hp because it was an older motor that was rated at the powerhead instead of being rated at the prop. What year is the 40hp your talking about? The 40-50hp johnson 2 cylinders changed a little in around 89. The powerheads stayed the same for the most part. Newer models had internal tilt trim offered.
Easy and cheap ways of making power on these motors: carbs(48hp or 50 same thing), reeds($45-$60), shave the head around .035"($25-$50). Those are three of the cheepest and easiest ways to get some more power. You can also cut the old exhaust off and have a new exhaust tuner built and welded on. Kind of a cow bell mega phone looking thing. But that would be a little more involved because you would have to rejet the carb. Porting is what will brings these motors ALIVE!!! There are a couple other mods that can be done to help this or that but then your geting into a full blown race motor. I love my 40hp evinrude and love the way it performs, runs with alot of built 70hps and gets better gas milage then them too :mrgreen: Alot of guys around here run them and love how reliable they are as long as you take care of them. Torquey motors,we run them with outboard jet pumps, lose 1/3 of the power but when the waters 3" deep in places you gotta do what you gotta do.
Here's a couple vids of my boat running a 60hp etec and a 60hp johnson (not stock :mrgreen: ). 1000ft drag up river.
this is the johnson: https://www.youtube.com/watch?v=aBeEiDHMNFk
This is the etec: https://www.youtube.com/watch?v=IqKipcwZXT0
