Could probably machine a cam from a steel round. Since the RPM is low the inertial force of the valves would be low you might get away with tired valve springs and plain steel cam. Other wise simply hardening it with a rich oxy might do the trick. COuld probably be done on a 4 axis cnc mill. If you got excited you could have it hard chromed. Just ideas.

Otherwise what about one way valves in the valve holes? If you're taking all the rocker gear off you could even drill right through.