Take them to a tractor store and have them tested, then rebuild the ones that are not to specs. There is only a spring and a precise hole.
Or buy 6, replace one engine and send the old ones out to be rebuilt, then put the rebuilt in your second engine and have the old ones rebuilt, or sell them.
If the engines are hard to start, run rough or smoke more than a Lehman usually does, go for it.
You should adjust the valves now too.
"...Tongue tied and twisted, just an earthbound misfit..."