This is a old idea but a good idea. The idea is to have stirling engine run off the waste heat from internal combustion engine. It what I call quick fix. It works by the have the cooling system warm up the stirling engine. The stirling will be connected on a shaft. It will not be touching the internal combustion engine so be easier to get them difference need and to control the temp difference. The advantage is that you get the control and perform of the internal combustion and the stirling improves thermal efficiency.

It is done.

Gas turbine powerplants often use the exhaust heat to power a steam turbine, adding power and efficiency.

- One does it only when efficiency is very important. More often, manufacturing costs speak against it.

- Heat exchangers are bulky, heavy, expensive, and may need maintenance. And two power sources are more difficult to drive than one. So you may find such a combination in a powerplant, but not on a motorcycle.

- Stirling engines aren't very useful. People take turbines.

- The added steam engine adds some efficiency, but there are simpler ways which are good also. For instance, exhaust gases are fed to an exchanger which heats the compressed air before it enters the combustion chamber. Just 2-5% worse than adding a steam turbine, but so much simpler.