Locate a fan over (or under) the finned engine head. Have the outer diameter of the fan consist of a band, similar to plastic computer fans. Have this band in close proximity to the primary belt and drive sheave. Have a bimetallic strip affixed to plate (maybe with cooling fins and/or deflectors) that is attached between the engine exhaust port and the exhaust pipe.

As this plate and bimetallic strip heat up the fan will move toward the primary belt. Eventually the outer rim of the fan will press against, and the fan will be driven by friction contact with the belt.