I don't think that there is any theoretical minimum, although they tend to run at non uniform speeds as they get extremely slow. Try reducing the friction and increasing the moment of intertia of the flywheel. [build a larger flywheel with more of the weight around the perimenter]. Our MM-6's
will quite consistently run at about 30 rpm.
I suggest that you make a free Stirling engine web page
on this server and show us what you have done.