Good work XYZ, if anyone wanted to get some in-depth information about Stirlings, a great resource is the "Hot Air Eingine Society". Sadly you must join to be able to read the stored files, but doing that does keep the spam away.
I have transcribed the three major Rider patents into readable english. There were over 30,000 of them sold, and it is a mature design once you understand its subtle improvements over the previous. Electric motors and the diesel ended them. You can access the plain english patents in "Ron Roberts stuff" in the files section.
He did not have access to silicone lubricants, stainless steels, and Teflon. Since there was a small amount of working-air leakage, and air-pump and relief valve added make-up air at the point of lowest internal pressure. He ran double atmospheric pressure, as any more than that ignited organic-oil lubricants.
Since you are running helium to avoid oxidation, you could eliminate the displacer rod seal by moving the internal displacer slug with external magnets. The cylinder can be non-magnetic stainless-steel, and a ferrous steel band around the displacer-slug.
If you don't want to raise the pressure because of the bellows limits, consider making two more of these to stack one above the other. Rotate the orientation of the flywheels so a vertical crankshaft can be shared by all three. Having three also makes the engine self-starting. You only have to heat the bottom one, and the rising heat can be shrouded to also work the ones above. You won't triple the power, but an 80% increase in power without using more fuel might be worth the cost and effort.
Consider making the hot and cold ends of the displacer cylinders two separate pieces with a ceramic section in the centers to slow the heat creep from one end to the other. I would make the regenerator half that size. It would reduce the thermal mass (bad) but it would also reduce the volume of the working gas (good).