Stirlingengineforum.com

This first image is apparently a variation on the manson engine.

It seems pretty clever because traveling through the long heating tube gives the air plenty of time to heat up, so the tube only needs to be just above the working temperature of the air. That means it doesn't have to handle the super-high temps.
The downside being the extra complexity in the form of valves and a camshaft.
Replacing the cylinders with a pair of rotary positive displacement pumps looks like a a viable change.
It sort of turns it into an external combustion gas turbine.
Torque would be lower but speed could be very high, and not lose momentum by reciprocating.
Just right for spinning a generator.

Using vane rotor like so may be better, as it's easier to make. The cam surfaces do need to different sizes though, with the exhaust one producing greater volume than the intake.

Aaaand I managed to screw up the previous image.