Re: Pistons for 1969 Merc 1000?
One caveat, older pistons (pre-'66 or so) have smaller wrist pins and won't work on your rods. If you should happen to find good used older pistons with the rods still installed, you could use them (the diameter of the big-end is same on all of the 100hp 90 c.i. motors).
That being said, stay away from the really really early motors with 2-ring pistons, the rings for those are getting quite hard to find. Any of the 3-ring style of pistons uses the more commonly-available ring set.
But the larger-diameter wrist pin pistons/rods are better, and sticking with a fully-matched set of pistons/rods is the best way to go.
HTH & G'luck with the parts hunt.........ed