LSM9DS1 A 9 degree of freedom Inertial Navigation Chip

Now that's an interesting idea. The flywheel shaft is 10mm long with a crank on each end, so there's not much room for the Puck. Perhaps a photo interrupter with a tab on the flywheel would be easier for RPM sensing.
The LTD (Low Temperature Differential) Stirling is right up against the wall to work using hot coffee and small design changes can move the needed temperature differential.
One idea to pursue is a pressure sensor tapped into the displacer chamber. This would allow one to study the differences in displacer designs such as size, materials and motion of the displacer.