Stunner, you still haven't said what size you're looking for.

I have had excellent success with smoke systems on engines as small as 91 4-strokes. Your biggest consideration when you get that small is having room for the tank and pump, so a plane with a sleek design may not work, but a wide-bodied plane, like a Stearman, or an Extra should be ok

Here is a pic of my GP Skybolt with a YS 91 and my home-made Smoker