I was looking for one some time back then was gifted with a simple oscillator which I used on my Willi/Falk project. I'm told that making one is a simple first machining project but have not tried yet.
How about a Graham VR1A single cylinder kit or a Regner Lily? You can look directly at Graham's website www.grahamind.com
or Jason at the train department carries them as well as the Regner products.
My BAGRS was converted to butane firing and is a fun little engine with all those whirring chains and sprockets even if a bit messy.