Heads for horizontal engines have oil return passage on the intake side while the vertical heads have the oil passage on the exhaust side. You can easily drill out the passage to make one work on the other.
Other than that they are same head, same valve sizes, springs. Now the 25/26/27/28/29hp versions have larger combustion chamber which will drop comp ratio on the smaller cc engines like the 16/18/19/20/22/23 if popped on.