I have no idea what's commonly available in the UK, but my friends here in the US play with old five-cylinder Kinners and Lycomings, Continentals, whatever. In my dreams I imagine restoring a Wright R-3350, but that's ridiculous. I would suggest--since this is a project I have also imagined doing--looking for a very basic radial, which means five or at best seven cylinders. You want the equivalent of a VW four, not a 911 Turbo six...