The answer is it depends....
What you'll find is that engine power = engine weight.
That makes the first thing to consider how well the boat would handle more weight.
How well your pontoon will handle more weight is going to be mostly about the diameter of your pontoons (more diameter = more weight capacity). If the waterline now is at about the half way point when looking at the side of the 'toons toward the rear, I'd be really hesitant to put a heavier motor on the back of that boat.
If the half way point on the toon is up out of the water, then maybe it WILL handle a bigger motor.
Older boats (say pre 2000) tend to run smaller diameter pontoons (18" -20"). They were designed with 25-40 hp motors in mind. Newer boats (lets say 2010 and newer) tend to run larger 'toons (say 25" diameter).
SO, all that in mind, 1. how old is your boat and 2, do you know the diameter of the tubes? or 3. do you know about where your current waterline is on the rear of the boat?
A Honda 50 or 60 will be the same weight as your 40, but I'm not sure you're going to pick up enough performance with either of those to make a swap worthwhile.
The Honda 75 and 90 are both 150 lbs heavier than your 40. Merc and Yammie both have 75, 90, and 115's that are the same weight as the Honda 75's and 90's. Any of those motors will have significantly more power that will be very noticeable when pulling.
Last, there may be some concern regarding the strength/type of construction of the motor pod/transom. If this is an older boat with an adjustable transom height, I would not mount up anything bigger than what you have now. If it's not adjustable, I would still want a look at how the motor pod is fastened to the floor of the boat. Some use just 4 bolts to secure the engine pod to fairly light cross menbers. Something like that will need to be beefed up significantly prior to putting any of the bigger heavier motors on it. -Al