Hi,
Not the worlds best expert on Stirlings. But I would not be real enthused about using aluminum for the cylinder or piston. The heat expansion would be awful for this application I think. Usually, you want material that doesn't soak up heat for those parts.
The one Stirling I have built, I used a 304 stainless steel, (material on hand), for the cylinder and graphite for the piston and valve. The stainless cylinder does not absorb heat fast and the graphite piston is lightweight and self-lubricating. And it has a super low heat absorption rate, preventing part growth and binding in the bore.
No rings of any sort be used. Sterlings make so little power that any drag in the powertrain will stop them dead. All you want is a very close running fit on the piston in the bore and that alone.
I would look at some Stirling models and pay close attention to the materials used in the construction of them.