Check out the Vortex fan by Heng's Industries. It's a great high CFM fan that fits into your existing 14" vent frame. I used one on my JayFlight and it works great. Installs in about 30 min without breaking the factory seal and can all be from inside.
Check around, it can be found for less than $50
You may want a vent cover to run I in the rain. I used MaxxAir II, again I'm very pleased and it installed easily without effecting the roof seal