An easy quick way to build a lively little engine is by using a pneumatic cylinder. Air cylinders can often be found at flea markets and swap meets, etc. The Surplus Center in Lincoln Nebraska has bushels of them. This engine has a simple spool valve and is connected with plastic tubing. The crankshaft bearings are from roller blade wheels. Skate boards, roller blades, and those little aluminum scooters are all equipped with ball bearings, and the bearings usually outlast the treads on the wheels so they are a good source for little bearings for your engines.
Although the cylinder on this engine is not a Bimba brand, I named it that anyway. The Baby Bimba shown below does have a Bimba cylinder. In fact the factory decal is still on it
It is a real little hummer, with a 1/4" bore by 1/2" stroke. I sort of suspect that is the smallest cylinder that Bimba makes.