The way I solved the 4 stroke problem was to weld and machine another cam lobe onto the opposite side of the camshaft this makes the valve open every revolution. I re-timed the cam to open the intake valve on the up stroke of the piston the intake valve is larger so it is better to use it for exhaust. The actual exhaust valve is not used and is installed just to seal the exhaust port the valve lifter is removed and plugged. The steam or air is injected through the spark plug hole by a steam valve of my own design that is operated by the cam lobe on the top of the flywheel. The reason I didn't use the exhaust valve as an intake is that that it would open in the direction of steam pressure and you would need too large of a valve spring to keep it closed. There isn't enough room in a small engine like this for a big enough spring. The combustion chamber is reduced as much as possible by welding and machining the cylinder head.
Yes there are advantages and disadvantages to using steam instead of wood gas. A steam engine is much simpler, it needs no ignition system and produces much more power at low or zero RPM there for it is much better suited for applications that need high starting or low speed torque. If you can produce steam pressure a steam engine will always start and run.
I hope this answers your questions, if not let me know.