Original Mikuni parts are nearly always stamped with their logo, a small square in the corner of a larger square. The needle will have a number stamped on it at the top, and I think that yours should show 5DH3. Repro parts have been known to cause problems.

Also, if you can, strip the carb and treat all of the metal parts to an ultra sonic clean.

I think that the cylinder needs to be removed in order to clean up the clogged decompression hole properly. If this is not possible for you to do, then you can buy aftermarket decompression valves that sit in the "spare" spark plug hole. However, the ones I have used are cable operated and so the handlebars get a bit cluttered!

I have never seen an original Mikuni "calibrated" part like a jet or slide needle that does not have the "square within the corner of a square" logo stamped into it.

I have seen aftermarket parts with the same numbers stamped onto them but they are often not the same, especially jets and slide needles.

Keyster for one, is famous for stamping the Mikuni size numbers onto their parts but will flat tell you that the calibration they use is "proprietary tuning" their engineers developed and likely not an exact copy of the original Mikuni part.