Hi and thank you for your question.
I understand the symptoms you are experiencing, starts and runs good, but won't accelerate past 25 MPH any more. It ran fine for an hour and a half prior to these symptoms showing up.
I do need to know which model you have to make sure I provide solid, reasonable advice for you. I imagine you would have mentioned this, but I have to ask. Are you getting any kind of warnings as to something being amiss? Also, does it make any unusual noises now that it didn't make before?
Is this ski new to you also? Just the way you said it ran fine the 1st 1.5 hrs made me wonder that.
A couple of generic things to look at is check the air filter and make sure it isn't wet from either water or oil. If it is, you will need to replace it. It will never flow the proper CFM of air again. The other generic item that is easy to check, esp with the air filter out, is the spark plugs. I would like to know what the tips look like. We would like to see all of them the same color. Also, make sure you keep them in order front to rear in case one is different, we will know which cylinder to further investigate.