Here's a twin flywheel mill type horizontal engine that uses a single crank throw. Valve pushrod operates a rocker arm that pivots on an 8-32 screw attached to the top of the block. Layout is similar to other mill engines on these pages, except valve moves vertically instead of horizontally. Valve motion for this design was barely enough because pivot point was too close to valve. Worker better as a result.
Second photo shows partly dissembled engine.
Plans at bottom show air inlet at front instead of side. Most dimensions on plans are approximate.