I'd think about a thermostatically controlled electric cooling fan. V-belts are inefficient. 90° v-belts even less so. And then your thermostat when closed is just adding back pressure to the fan, which is already sucking power when not needed.
They make kits with everything you need in both centrifugal:
Corvair Specialties - Products:Â Corvair Street Electric Cooling fan
And axial:
http://www.corvairspecialties.com/fankit.html