Balsa wood is a good heat insulator to start with! Also, looking at how vacuum flasks work, its possibile to apply that with the whole engine block (or just the subsystems that generate the most heat). The heat outlet can be directed away from the cabin.
I'm not sure how the research of silicon nanowires is progressing, but you can use them as thermoelectric converters to convert the heat back into useful energy. Just wrap the engine up with them
