If you get resistance rotating the cam one direction, then rotate the opposite direction to see if the tension on the rockers can be relieved to allow removal. If you reach a point that you can't rotate the cam to relieve the tension on one or more of the rockers (if a valve(s) touches a piston) then you could still pull the bottom half of the lower case, then remove the crank and pistons from below. This would eliminate any interference with the valves.