If I were going to add a fan to the inside of my smoker... I would probably keep it as simple as possible and only put the components that actually HAD to be in the chamber, in there, due to heat , smoke, and kreosote. In this case - it would be a blade and a shaft...
The general concept would just involve drilling a hole , adding some sort of heat sink or heat dissipating spacer , and a bearing - then sliding just the blade and fan inside, and turning the shaft externally with whatever method was most appropriate for the application... ( pulley / belt, DC motor, gears , etc ...)