If your RC valve is not working then I suggest a new CDI unit. My recommendation here is an OEM unit, not one of the made in Thailand aftermarket ones. We've tried them all and although some actually seem to make good power, none of them last more than ten minutes. At some point I may try an adjustable ignition, but got more pressing matters right now.
The NSR250 carb needs a bit of work to get to fit. We fit made a kit a few years ago, but ceased offering it when supply of good used MC21 carbs dried up. They work very well and give more power and over rev. Not sure how they'd work on an otherwise stock bike, but if a pipe is fitted then they really fly!
Getting (real) power in the low 30's is not too difficult as the engine is severely restricted by the pipe and carb. They are designed for low end rpm, town use. Not racing.
Hitting 40hp reliably may not be easy without lots of mods and special fuel. I spent many hours on the dyno getting an old NSR150RR (pre SP) up to 39hp on 100 octane race fuel, a PJ34 carb and standard CDI. It hit a genuine 120mph at an airfield circuit. Would have gone faster if we were actually speed testing it, not racing it.
On the dyno one day, and AP Honda (Thai Honda Company) came to test the (then) brand new NSR150SP. It hadn't actually been released at this point. They had to come and use our dyno, which was cool as we let them use it for free provided I could sit in on the tests. Anyway, they had about 10 cylinders/heads & three or four pipes to test in the day, but none of them would exceed the 39hp that I got out of the RR (we were overlaying the graphs). But then they played the joker and plugged in a different CDI. Instantly 40hp. BUGGERS!!! There was a many pursed lips as I interrogated them about this CDI, and they packed up and left. Make of that what you will.
So, if you want 40hp then you need to spend many hours modifying the motor. It won't get there without excessive use of the grinder.
Wiseco don't have a piston compatible for the NSR150SP. And our 59mm piston is designed for the 300 kit and I own the forging.....or at least I paid the $$$$ for the bloody thing!