First, make a mark on your engine case next to the flywheel. It doesn't matter where you put the mark so long as you can see it.
Method 1: Use a piston stop
Screw a piston stop into the spark plug hole (the rope trick will not work for this). Rotate the flywheel until the piston hits the piston stop. Make a mark on the flywheel that lines up with the mark on the case. Next, rotate the flywheel in the opposite direction until it hits the piston stop again. Make another mark on the flywheel that lines up with the mark on the case. Finally, make a third mark that is exactly halfway between the two marks you made on the flywheel. This third mark shows when your piston is at TDC. (It is helpful to make this mark darker or label it "T" so that you know which mark is which).
Method 2: Buy an ignition timing tool
An ignition timing tool like this or this will make your life a lot easier and your timing a lot more accurate. If you screw one of these tools into the spark plug hole, you can see (at very precise measurements) how far the piston is traveling. When the piston has reached its maximum distance, the tool will show it, and you can make a mark on the flywheel that lines up with the mark on the case, just as above. Of course, many mopeders are too cheap to buy these tools. If you fall into that category, you can make a homemade (but less accurate) version for under $5, or you can combine a hollowed out spark plug with a cheap dial indicator for what would hopefully be an accurate but half-price version.
Method 3: Use the screwdriver method
Make a mark on the cases as explained above. Then take out the spark plug. Stick a screwdriver or other thin and long implement into the spark plug hole (a drinking straw also works well). Rotate the flywheel. As it moves up and down, the piston should push the screwdriver up and down. There will be a point where the screwdriver is pushed out the farthest. When you think you have found that point, make a mark on the flywheel that lines up with the mark on the case. This is the TDC mark.
Of these three methods, Method 3 is the least accurate. There is a period of "dwell time" at the top of the stroke (near TDC) where the piston moves up and down very little but the crank is still rotating at the same rate. This makes it difficult to judge when the "farthest out" point is. If you need to find TDC to set your timing, method 3 will work just fine for getting the timing close enough to make your moped spark and run (and probably run well) but if you're aiming for maximum performance and maximum accuracy, either use a piston stop or get the correct tool.