Something that takes either a large glow 4-stroke or, even better, a gas engine. What kind of biplane are you wanting? Aerobatic? Sport? 3D? What are you currently flying? Just based on what I've read so far, I'm hoping that you're not trying to step from a trainer to a smoke-system-equipped advanced aircraft.

The smallest gas engine that is readily available commercially I believe is the Zenoah 20cc, which is 1.2 cubic inches, equivalent to about a .90 size glow 2-stroke. I think that the smoke actually starts being a viable option, however, when you get up toward the 50cc range, or higher. It's not just the engine that you have to consider, but also the extra weight and equipment that comes with a smoke system.

Go find yourself a giant scale biplane, if you wish, and put a 50cc gas engine and smoke system in it. That's my recommendation, assuming you have the experience and skills necessary for such a project.