Having restored the full sized Ericsson sterling and built a Denny model, you may want to try using a buna or better viton cup seal on the bottom of the power piston to compensate for a larger (sloppier) piston fit . On the larger Ericsson, it uses a leather cup seal which inflates during the power stroke and allows air into the cylinder during the down stroke. This seal design would work well on an alpha configuration where the power piston and the displaced are similar sized. You could also get away with an aluminum air cooled cylinder and power piston depending on how much heat you are trying to remove. The larger the delta T you’re working with, the more power you’ll get to overcome friction at the downside of needing higher temp seals and such. If you’re looking to make a coffee cup type of stirling, then you’ll need a really precise bore/piston with no rings needed as others have stated. My Denny Ericsson type model used a cast iron sleeve which was honed and single o ring piston which worked well. Trick here was to have an adjustable ring to variably squash the oring to get Optimum fit.