I know this thread is long dead but living in a country that also has 34 bhp laws its damn funny for one brit to ask a question then have only one other european answer and a dozen americans give answers suggesting DIY solutions, i can't help but post.
First of all the "ninja 250r" or the gpz250 as it was originally called hasnt existed in eruope since....since....heck i cant remember that far back but its almost impossible to find one that works in europe, the closest one might find is a Kawasaki Ninja ZXR250, which has an in-line 4 that produced 45 bhp, so not much luck there.
Next of all bikes like the cagiva mito and rs125 which are almost 34bhp when de-restricted are nothing in comparison to a 600cc bike when restricted down, a large majority of the torque from a 600cc engine stays and the power is lost in mainly the top end.
having ridden 34 bhp 600's before i can tell you this, you lose 2-3 seconds on your 0-60 times and your top speed will hover in the 100-110 range, more than enough for one's first bike; performance that is very similar to 125cc two strokes, except that your acceleration at higher speed is excellent, and when riding fast, you are less annoying, you look and sound nicer to passer by's, and the bike doesnt feel like it could easily explode or go air-borne over 80mph.
With kawasaki theres two options they offer: First is a kit that blocks the throttle spindle which is usually in FI models and second is for older bikes whereby the restricting rings and spacers are installed in the carburettors. The kits are usually done very well and the restriction is not immediately evident, throttle response, power delivery and fuel consumption are quite good. Also restriction has to be done officially otherwise any squid could shove socks in their airbox, go do an inspection then chuck them in the wash and then head off to have a big accident down the road.
Overall its great because the squid population gets kept under control (they are just the 16 year olds on 50cc scooters and "bikes" who simply disappear when they turn 18 and buy a car) plus buying an old zx-6r ninja and restricting it is kinder to the wallet in longterm and possibly short term too. Another advantage is you learn how to handle a big bike's weight, handling and braking before you can get carried away with 110 bhp.
p.s. the cbr125 is lame to the nth degree whereby n is a very large number inversely propertional to the size of the useless bicycle tyre honda calls the rear wheel, plus its a mono-4stroke so you get 13 bhp, it was created to make money for honda, period.
