The trim cylinders on my '77 Merc IL6 were sealed with orings. The piston was sealed with a teflon seal, which required a special tool to install.
If she is leaking around the trim rod, you might go on crowleymarine and see if they have an exploded diagram of the trim cylinders. Those diagrams often give sizes for the orings. You can then order them online. Mercury may also have a rebuild kit part number, which you may find some NOS.
