Are you talking about a Flatback?
I've drove a FB with a 90 hp on it and while not a rocket, ran fine, the way they were designed to be run.
Seems most members go 150-175hp as the norm and some have gone as high as 225hp, which they admit is way too much power. The boat gets squirrely on them above 40.
If you're talking about a 12degree Osprey, then 135 is probably too little.
You can look in our Gallery for spec sheets on HP ratings.