I would mount the rods first and fully assemble the main case halves with the complete transmission and cam chain installed. Then put the pistons on, with rings in, once I have the engine right side up. That avoids any damage to the pistons dusing case assembly. Just put one circlip (ALWAYS use new circlips) in each piston, on the side towards the center of the engine. Then line up the pistons and put the pins & 2nd clip in 2 & 3, then 1 & 4.
Remember to gap the rings. This is critical, and purchasing a gap file tool is worthwhile - they don't cost much. It's quite hard to file the ring ends flat and level without the tool.
The two similar rings have different profiles. One is basically flat where it will contact the cylinder wall, the other is bevelled slightly. The flat one is the top or compression ring. The bevelled one is the 2nd or scraper ring, the bevel faces so that it scrapes oil downward. The bevel may be hard to see but if you look at the end with a magnifier it should be visible.
Don't forget the base gasket.
Block 2&3 pistons up with a bit of wood or a screwdriver handle, something softish. Slide the cylinder block down gently onto the pistons, carefully aligning them. There's a conical section at the bottom of the barrels that makes it quite easy to slip the rings into the bore by just working them a bit with your fingers. Once all the 2&3 rings are in remove the wooden block and rotate the crank so that the cylinder block moves down with 2&3 while 1&4 go up into their bores and then work the 1&2 rings in. Then fish the camchain through and slide the cylinder block down all the way onto the locating pins.
