The Famous Blackbone 2 Speed System for the HPI BAJA!
 
The kit contains:
HD steel clutch bell (15mm shaft diameter)
HD 2 speed custom layshaft with one way bearing system
2 speed Alloy Spur gear carrier 
Get the best of both worlds for your Baja, Groud breaking launch and hit second gear for that top end speed you are looking for. Don't choose between torque or speed. Get both with this BBRC two speed.

Gears are available via separate listing, this is just for the 2 speed system. 

IMPORTANT GEARING ADVICE:
Don’t set the difference between first and second too great or you’ll notice smaller engines will struggle to pull second gear. a Maximum of 4 pinion teeth for the jump, eg 18/56 and 24/50 will be rubbish and no point buying this system for bad performance
For most tuned zenoah style engines, start with 18,19 or 20 for 1st gear and go for 20, 21 or 22 for second gear, youll get the potential from the engine and the transmission this way.
My advice on 24/50 is that only really for 40s or twins. 23/51 is plenty for second gear on even a very powerful 34cc.