The thing on top of the piston is called a baffle. If you look at the inlet port in the liner you'll see it's just a machined hole with no angles on it like you see in modern engines. This means the fresh mixture tries to just blow through it and straight out the exhaust so the baffle is there to deflect the mixture upwards. If you can remember what it's used for then you won't fit the piston backwards as happens sometimes. There's a matching slot in the cylinder head so it can only fit one way too. If the glow plug is offset over this baffle slot then you have to be very careful that the plug isn't so long that it can protrude into the slot and foul the baffle. This particularly holds if you use an idle bar plug.