Have a search on youtube, several smaller ones there such as this sitting on the end of a pencil.

You could probably say the same about my Warco 280 but my stirling engines work OK. It does become more of an issue as the parts become smaller if you think of the runout in relation to the diameter then fairly easy to see that 10microns on my 40mm cylinder is not going to be that noticable but it is 10times greater on a 4mm piston and 40times greater if that piston is on a 1mm dia rod.

Runout can be compensated for by doing as much in one setting as possible, using collets or custom turned split bushes which should be better than the average collet’s runout. It is clearances and alignment of parts as much as concentricity, lapping the power piston is going to be the critical fit if you are making alpha or gamma types. it gets a bit more complicated on the beta types where the power piston and displacer are on the same axis so displacer rod needs to pass through the power piston but they have been Ok for the ones I have done.

I’ve not really looked to see what small barstock designs might be out there as I prefer a scale look but there should be something and it can always be scaled down.