oh, and maybe, just maybe depending on where the speedo pick up is, altering the gearing may affect your speedo reading. I suspect yours will be off the front wheel so wont affect it.
Mine on the other hand is on the front sprocket and adding a tooth there (equivalent to taking 3 off the back) brought my speedo more to true...i.e. instead of 10% over reading, its now 3% over reading.
I remember nearly 3 yrs ago when I was choosing a 125. Two dealers in mind, one was saying a bit like GUNS, go for the 2 stroke for power comparison, and the other saying similar to S24_SDJ, but they said about every 12k for a rebuild. In the end I went for a CBR125R.
But same as what Geoff’s saying, I would go for the test and get yourself a 500/600. I found power wise it was quantum leaps compared to a 125.
Not just that, 2 yrs ago when I passed the basic, I was tempted to go for a black ninja 250 that would’ve cost just over £4K new, but my current bike (ZX-6R) in the same showroom, was a grand less but with an extra 60+ BHP! (2002 reg).
The moral of story is the same price you would pay for a brand new 125 or 250cc bike, would be a lot less power even against the bigger bike albeit it has age.