You do not need an airtight seal in a piston and cylinder to use air as a working fluid. In fact, when lowering water buoyancy, the force that pushes objects out of the water, then an object sinks, and as soon as the air is out of the water, then the thing rises again. Thus producing an up-and-down movement just like a piston in an engine.

Changing the buoyancy force could be done by adding salts of different kinds, e.g., table salt or potassium salt, or the water could be replaced with oils. It is only a matter of experimentation, an open mind, and creativity.

The absolute best about this solution is that nothing needs to fit together at high tolerances, so everyone with access to a hammer, saw, and drill should be able to make this.