Hi Heinz.... come to the dark site... move to the Hobie 14/16 category within this forum. Both, the solo sailing and the sealing of masts have been discussed there.
In short... Identify the leak (most likely it is the foam plug under the mast top pulley) Drill out the rivets. Use a new foam plug if required or reseal (silicon) the old. After a good breakfast pop those rivets back in place (oh… use the sleeves to make the rivets watertight).

Patrick