OK - I don't get it. In the photo of the bike (and the photos of the internals of the cylinder, piston and motor) it looks like the transfer port is at the back and in the inlet port (the lowest port, that is uncovered and joins the carb to the crank case at the top of the stroke) is on the left (near) side of the bike. Viewed from this side, the steep part of the piston crown is to the rear, so the piston ring gap was always in line with the inlet port? Or is it an optical illusion? Or maybe the rings stay above the inlet port, in which case which port does the ring join line up with?
Maybe the piston in the bike when you got it was incorrect. When you peg the rings so the joins don't align with a port, make sure you fill the original peg holes. Or just use the dremel to remove the half of the original peg projecting into the ring groove.
[Edit: I just can't imagine you'd have to slide the ring gap across a port to assemble the motor!]
Cheers
Leon