The fusion side of this is totally theoretical, but if you could have "Cold Fusion" of He3 without magnetic plasma containment (possibly using some type of exotic metal lattice and a "resonate" frequency) you could get a fusion engine that produced much less waste heat than a gasoline engine for the same power output.
You primarily get energy from proton generation (potentially 95% of the reaction can be harnessed directly from this, but current experiments have only gotten 70%).
For any waste heat I imagine the reaction vessel being wrapped in the new polymers which convert a significant portion of that waste heat into additional electricity.
http://sciencenordic.com/plastic-can...at-electricity
The limit of efficiency and the amount of waste heat produced would all depend on the details of the plastics heat to energy conversion. I would assume at low power generation levels the heat lost would be negligible. At higher levels it would be higher depending on whatever limitations there are in the polymer but even at the highest levels it might not exceed 30% of the total energy. Most gasoline engines lose 80% of the reaction energy as waste heat IIRC.