15 > 25hp upgrade, will i need reinforce my hull/transom?

I have two motors, a 15hp mercury (not tohatsu 18) and a 25hp mercury. both 2 strokes and short shafts. i have been running the 15hp on my Quintrex Dart 355 2008 model and when it has more than 2 people and floorboards/gear it is too slow, also is a little too slow for watersports. the manufacturers recommended max hp is 15.. and a max weight of 58kg (the 25hp is 50kg). the boat looks very solid as is, but would just like some other opinions on this. the boat is only used in private waters on my property so legalities are not an issue.

Cheers in advance!

its not the legality you have to worry about, its structure. You're looking at nearly doubling the recommended horsepower, which is probably more than the boat can handle. It might be fine for a short time, but between the thrust force and the vibration you're gonna crack your transom.