Years ago I obtained a used "one cylinder needs repair" 115 that I wanted to repair and put on my GT, because if I remember correctly only the jetting and/or rpm or timing limit was different between the 90's and 115 "towers of power" at that time.
Well, I have a friend who our families used to snowmobile and camp together with and he was also with a local sheriff's department at that time (he eventually became the sheriff, but is retired now, but their department always and still patrols the lake we like to frequent). He strongly cautioned me at that time that the DNR in Wisconsin really goes out of their way to "crack down" on everything. It was common at that time to just put on a 90 cover or stickers - but he said they frequently would stop you for a "courtesy safety spot-check" (especially if you were a fast(er) boat on the lake) and would mark down the serial numbers from the motor and have them checked (I saw them do this a few times on that lake too - and not just the "required safety equipment on board" checks - even recently within the last few summers).
So if you modified a 90 to be a 115, you stood a better chance of not getting caught than starting with a 115. He said they were tough enough on just people who modified and got caught (and I remember seeing newspaper reports of these "inspections" through the years on our local flowage), but if you had an accident - they threw everything in the book at people, not to mention the insurance and getting sued risks!
So as a result of not being brave enough to take the chance, my 115 still sits on the floor in storage which I probably will just sell some day whether I repair it or not. I always envy but used to be amazed when I'd see so many GT's with 115's - but it seems to be OK as long as you never plan to run in Wisconsin (WI side of Mississippi seems to be ignored? (maybe better chances if you're not a WI resident?)).
The real irony - if I understand it right - is that the engines at that time were rated at the engine head instead of the prop like they are now (hopefully I don't have that reversed) - so the old 115 "towers of power" were actually only 90 HP at the prop anyway in reality...but good luck arguing that little fact with our DNR! (remember last year or the year before when someone rescued a deer that even the DNR admitted was kind of a unique situation - but greatly angering the public and even the Governor, they killed it, and told the public and even our Governor after the outcry that "THEY" make the rules...I've met some wonderful DNR people, but I've always heard of some who have "a real power ego problem" and unfortunately that power seems to "reign supreme" in many instances)
