The marzocchi forks arent very responsive to their screw adjustment. If you are a relative beginner id find it a tad unusual the forks are too soft for you. Dont worry if you are using most of your movement over smallish obstacles, trials suspension is a fair bit softer than road or motocross suspension.
However a bike that age could be more than due an oil change, old oil will reduce the damping and make it feel soft. Can be a bit of a pain on the marzocchis. If they still feel to soft for you you could make up a pastic or alloy spacer to go between the top of the springs and end cap to preload the spring about 1/2 inch (if you dont fancy buying new springs), although to reiterate, ive never really known anyone to want to harden the marzocchi forks, beginner and expert alike.