The best way to clean the decompression port is to take the whole cylinder head off. This is much easier than it sounds.
Here's what you'll need:
A metric socket set,
A can of Wynn's carburetor cleaner,
A wire brush.
The cylinder head is the part that your spark plug screws into. It is held on by four bolts.
1. Disconnect the spark plug wire.
2. Disconnect the cable that attaches to the decompression mechanism. This should be the only cable attached to the cylinder head. Pay attention to how it is connected!
3. Take the cylinder head off by loosening the head bolts. These are the bolts in the four corners of the cylinder head when you look straight down the spark plug. Loosen them all at once rather than one at a time (like when you change a tire on a car). Don't lose the nuts or the washers [duh].
4. The cylinder head should pull right off. If your cylinder head has a gasket underneath, make sure not to damage it and remove it too. Take the spark plug out.
5. Scrub the cylinder head with the wire brush and carb cleaner until it is shiny. Clean the decompression port mechanism too.
6. Turn the flywheel on your moped until the piston pushes all the way to the top of its travel (so it's flush with where the cylinder head bolts on). Scrub the bejesus out of the top of the piston and get it nice and shiny. Don't scratch it!
7. Reassemble everything in the reverse order you took it apart. DO NOT overtighten the cylinder head nuts (very firmly tight, but don't strain). DO NOT overtighten the spark plug (screw it in by hand as tight as you can and then give it another eighth of a turn with the socket wrench). Work all the cylinder nuts on at the same time (once again like the way you're supposed to tighten lug nuts on a car wheel).