I wonder if this has been done before?

I'm currently playing around with the design of an alpha-type Stirling engine in order to power a 'diesel-electric' model train. I'm using two model aircraft cylinder/piston assemblies, one larger than the other, to form the hot and cold cylinders. These will be mounted at 90 degrees to each other and will have their connecting rods tied to a common crank pin on the flywheel. The rotation of the flywheel will be coupled to a small generator which will supply power to the traction motors, via a standard electronic speed controller of the type used in R/C model cars, boats, etc. Nothing novel so far, right?

Where I think I've been a little clever (though maybe someone thought of this before?) is to run the hot cylinder from a butane gas burner as it's heat source (actually, I'm thinking maybe more than one of them, of the type used in butane 'pencil' soldering torches). The cold cylinder is enclosed IN the gas tank which powers this burner. The tank will be formed from silver soldered copper, enclosed in an insulation jacket, with the flat bottom of the tank forming the cylinder head gasket for the cold cylinder assembly (I tried that with a flat plate of 3/4mm copper and it works fine).

Since the consumption of gas will reduce the volume contained within the tank, and the gas cools rapidly as it expands to fill the now larger volume available, the end result is to actively heat one cylinder of the engine while actively cooling the other. This increases efficiency fairly markedly while requiring no energy take off from the mechanical output to achieve it. The fuel tank will get cold anyway, so may as well use the cold for something useful, is what I figure.

What I'm wondering is if anyone has a good design for a regenerator suitable for use with this? It needs to be capable of screwing into the 'glow plug' hole of each cylinder head, thus can't have an internal diameter larger than maybe 8mm without leading to wasted volume, lowering efficiency.