ok.. I see in the photo it's clamped on. This style is easy and convenient but it's hard to get a good seal. While the engine is disassembled and after parts are clean, do whatever it takes to fit that exhaust pipe to the port properly.
The clamp might be fatigued and will not tighten enough. File away some metal so it can close farther. The slot cut into the end of the pipe may be longer than needed and exhaust leaks from there. You could hard-solder or braze or weld the portion of the slot that extends wider than the clamp width.
As far as cleaning, i think the best info i found was from a cooking website... the carbonized gunk thats burned onto iron pots and pans is near the same stuff. One page recommended applying oven cleaner, put the pot in a plastic bag to delay the cleaner drying out prematurely. Leave overnight and repeat if needed.
I've never seen any part of a stock moped exhaust made of aluminum so the oven cleaner soak is probably safe and worth a try.
Take the pipe apart as far as it's assembly allows and make a decision based on that. All you need is a clear pathway all the way through the pipe. Partially blocked passages are the target, not the Good Housekeeping Seal of Approval.