Are you ordering one or what? If you already have it see if it lines up. If it does screw it in. If you're going to order just look for a 90mm fan instead...
edit: @Atomic Rooster. I took apart one of my arctic cooling fans and tried to mount it on a heatsink that didn't have holes for screws. I broke off the little mounting bracket of the fan and ziptied it back but upside down. It had some plastic things sticking out, I tried to put those down the heatisink fins. The size was perfect. Unfortunately the way I ziptied it prevented the thick part of the plastic to lock in. I ended up breaking the fan. It was good practice though.