AFAIK (edit
) it's a big problem to transform heat info other type of energy efficiently.
I can't propose ready to use device - only just my random thoughts. I've read something about hybrids with turbines making power for electric motors. Maybe you could borrow some ideas from them
You could run small steam turbine, produce el. energy, then run an air compressor (and use the heat again), store the compressed air and use it to push pistons, let say, every second cycle. Probably you would have lots of additional issues on the way of course. It would be very complex and expensive also. Cost vs. gain could be too small to think about it seriously but hey! it's an idea
PS
Don't take me too seriously please.
BTW
Maybe it would be even better to simplify the setup. Put turbo, place high rpm high compression pump instead standard compressor and pump the air into accumulator for compressed air injector. You coould also use a heat from exhoust collector, turbo and pump. Sorry for wild imagination