What is the heat signature of a piston engine, relative to a jet engine?
Here is a color IR image of what I believe to be another F6F, on the tarmac shortly after landing:



As you can see, the engine cowling is very noticeable, as are the tires (from touchdown and braking).

Here's a similar image of an F/A-18 performing a runup test prior to takeoff. This jet is longer and larger than the F6F (though if you've ever stood under a Hellcat you'll notice they're not exactly small planes), and as you can plainly see, the entire back half of the aircraft glows white-hot:



So clearly jets are more visible targets for an IR missile, though a prop plane wouldn't be that hard for an IR seeker to find.

The biggest issue relating to getting a good IR lock and track is contrast. Against a blue sky, almost any airplane is readily visible to the missile's seeker. The biggest problem for combat aircraft in early engagements was firing at a target below the horizon (so-called "look-down, shoot-down"). This was a problem for both radar and IR technologies. For IR, solar heating of the ground produced high enough temperatures that the aircraft could not be distinguished unless it had turned its tail to the shooting aircraft and was showing its 600-degree exhaust plume. Nothing on the ground underneath it would be that hot. Prop planes, while operating at lower temperatures, would still be warmer than the ground, and unlike a jet the aspect angles at which this heat is most noticeable is around the front half of the aircraft.