It's up to you what you spend your money on but chambers and/or cans aren't worth anything really power wise on the '18 or '21 especially on the road (i.e they give about 2 bhp at the top and cost more mid range). They do sound nice but I know of a '21 that saw 66bhp on the stock chambers
A lot of the '21 restriction is in the wiring so if it's an increase in power (rather than just looking nice) you're after spend some of your 23000 yen on some tools and follow this.
You'll have done nothing except let the bike work properly in 4th, 5th and 6th so everything should be fine but do a plug chop* in 4th, if that's OK try 5th then 6th just to be sure as some hero may have jetted it down in the past.
Then mod the airbox, Tyga do this to theirs, other people prefer 4 1" holes in the top, mine has got two 1 1/2" holes in, whatever, the "problem" is the inlet in the airbox is not as big as the carbs, it doesn't take a rocket scientist to see this cannot be good.
The reason I say do this seperately is it almost certainly will require rejetting now as you are now allowing more air in, this is probably now the time to consider fitting the HRC (or JHA) jet kit.
*So, at the end of a long enough to get up to max revs in the gear you're checking it in, pull the clutch in, flick the kill switch, and coast to a halt. Remove the front spark plug and check the colour of the electrode. It should be a light tan colour if your settings are good. As it's probably standard it won't be loads too rich (black) but we're looking for grey or near white indicating it's too lean which means a gentle (Andy ) ride home and some larger main jets with some more of your 23000 yen.
Plenty to read about plug chops, jetting etc. on the forum and in the tuning bits so have a search.
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