First one at the top of the page. I picked up one of those at a local store several years ago. Advantage is that it is very thin and allows more room to look through the spark plug hole. They are here in the States, you may have to look around. Best to try and view the piston in relative darkness, make a “tent” over your head and the engine if you are outside in daylight.
What you are looking to see is not only the color, but the shape and size of the combustion pattern on the piston.
Black all over = rich
Small brown center/black outside = still rich, but better.
Brown circular pattern reaching to within 1/4 to 1/8 inch of the piston’s edge = pretty much prime territory.
Brown all the way out, any lighter/whiter or “pecked” looking spots = Don’t Go There!
+1 for Chris’s advice.
+1 for Christaan’s comments too.
If any of the running issues are happening at part-throttle, it is time to research needles for your carb.