i have alot of experience tuning the nsr125 and mine happened to be a alcast but a msr125f

the restriction on these (uk) is down to the exhaust , the gearing and the inlet manifold

first is the inlet manifold it has a plate between the rubber manifold and the reed valves this has a smaller hole in it restricting the flow of mixture and creating turbulance this puts the hp down to 18hp so bout 70mph with standard gearing 80 with less teath on the back.

the second is the exhaust this has a ring welded into the inlet stopping the effectivness of the expansion chamber best bet is to drill wer the welds holding it are and tapping it out

the last is the gearing with a full power engine you will get 100mph from it if put a rear aproket with 44 less teath this will increas the top speed to bout 110mph u can put a smaller one only but only any use if u live on a very flat part of the world.

the bike could be restricted by any of these means or even all like mine was.

you can increase the top end even more by putting a cheap s and b (kandnstyle) filter on and increasing the jet sizes the exhaust is fine just cut the can of the end and put a lighter one on the end i used an old 250 trials bike one.