H25 OB motor spec
The difference between a '9.9-HP' and a '10-HP' outboard is that one is called a '9.9-HP' outboard and the other one is honest. '9.9' is a legal way around a 10-HP prohibition on many inland lakes. Nowadays you nearly never see a '10-HP' outboard any more.
It was once true that Johnson/Evinrude and Mercury 9.9-HP motors and their corresponding 15-HP motors were the same blocks; the 9.9's had detuned carbs. More recently I did some research while working at WM and found that the new 9.9 and 15 Mercurys are entirely different blocks, so that's no longer true; and of course most of them are fuel-injected now.
I know several people with 15s on their 26- and smaller boats; and they're fine. For a retractable outboard bracket the usual concern is for weight, not thrust. Most newer 4-strokes are too heavy, like by 33% in some cases, compared to older 2-strokes. Your MW is likely a 2-stroke and if it's enough under the weight limit of your outboard bracket it's fine. My ancient Garelick OB bracket, original to the boat, is rated at like 88 lbs or something. This necessitates a 2-stroke and prohibits a 4-stroke. I'd love to have 15 HP as the H25 is not particularly light compared to 'trailerable' boats of the same period; but I just got an Evinrude 9.9 2-stroke, long-shaft, and that's like the perfect motor for it. An 8 or 7.5 (like the excellent old Evinrude Yachtwin, if you can get one with long shaft) is absolutely as little as I would recommend.
If the OB bracket is in the original location (mine is not) a 20" shaft (= 1 extension) is absolutely mandatory. An anti-cavitation plate, like the Doel-Fin, can be of some help. Powering in through an inlet with two guys lowering sail on the foredeck will bring the prop too close to the surface for comfort.
I would advise anyone with an H25 to have both electric start and remote controls as it's a nightmare having to reach down and control the motor over that transom!