An attic gable fan is a good idea for durability, but I think it might not be so good a choice in terms of excess power consumption. For a modest sized engine with a likely oversized radiator, we don't need much of a fan. Maybe you could find a very wimpy attic fan, or perhaps some other modest power AC fan.
I do like the AC fan idea better than DC, the temperature switch will last much longer on AC. Breaking contact on DC causes arcing and one-way metal deposits that shorten contact life. Induction motors are also typically far superior in longevity than brushed DC motors.