If you have clearance in the domes for the current pistons don't mess wih them. Check it with a L shaped piece of solder down the spark plug hole.make sure it goes over to the edge of the dome.
Also, if your current cylinders were at 66, you could nickasil them, but you are past that so it is time to find a set of cheap stockers on low bore or find something aftermarket. You already have many of the mods to support a ported motor.
Quit running marine oil. As stated it is for much lower operating temps and not suited to the abuse of most atv's. Try some dominator, anything castor, klotz, something for ATV's or dirtbikes.