Fitting your 125GP bike
It's really important that you're comfortable on your bike if you want to ride at your best. This includes everything from handle bar placement, lever (hand and foot) heights and placement, foot rest placement, and seat placement. One of the most frequent comments you'll hear about 125GP bikes is "they're so small!". Well, if your bike weighed only 71kgs, it would probably be a similar size too! Because of this, the ergonomics of a 125 racer is even more important than normal. The key to getting a tall person to fit the compact chassis of a 125 is to stretch out the controls: handle bars, seat length, seat height, and foot pegs.
Here's a few ways you can achieve this with little effort:
1 ) Extended handle bar mounts. These can be forward extended (photo 1) and/or raised(photo 2) with the right type of mount.
2 ) Seat subframe. These can be made or purchased to be higher (photo 3 top) or lengthened (photo 3 bottom). Photo 4 shows Brian Hardaker's RS125 which has an extended seat frame and a raised seat using a block of hard foam. Note the white block of hard foam added to the rear of the tank to keen weight rearward. Photo 1 showing the forward extended handle bar mounts is the same bike.
3) Extended foot pegs (rear sets). Photo 4 shows how this can be done by adding a mounting plate between the foot peg hanger and the chassis. These can have multiple holes to tune the foot position to be most comfortable.
Advantages of being a 'larger' rider
Whilst it's traditional to think that if you're a tall person you've got no chance of competing up front of a 125 class, from what I've seen this isn't exactly true. With a properly set up bike, a taller rider has a few subtle advantages over a shorter rider:
1 ) A taller rider may have an aerodynamics advantage. The longer back of a tall rider, along with a stretched out riding position (see above) provides a smoother transition for air to flow between the screen and the tail. A short rider provides a quick transition which isn't good for air flow and means flow separation and drag from turbulence.
2 ) A larger rider may have a braking advantage. Long arms allow a rider to keep their weight rearward under heavy braking for which a 125GP bike has in abundance. This allows the rear wheel to be kept on the ground, additional stability and even later braking than the smaller riders. This can be seen time and time again in the 2015 Moto3 championship where Jack Miller (70kg & 175cm) continuously out braked Efren Vazquez (56kg & 160cm). Whilst Vazquez undoubtably had an acceleration advantage, his short arms let down his braking which lead Miller to nickname him the t-rex.
The main point
Racing a 125GP bike is not forgiving. With a high revving and compact power band, they reward smooth and consistent riders. Particularly on a club level, I truly don't believe that not being a racing 'jockey' is such a disadvantage. A well set up bike and a good rider should easily over come this often touted disadvantage many use as an excuse not to enjoy these highly satisfying racers.