Yami-racer

Hit there, i have a modified Aerox, it starts and rides but after a while loses power, it will usually reach about 60mph but can die down to about 10mph (which can be dangerous) this was after just 3miles of riding, anf then also i was on my way home and its died asif i turned off the ignition, i have no idea what the problem could be, i was thinking maybe a sparking problem?? I desperately need advise as this has really baffalled me :S

I have a 2001 Yamaha Aerox that i have just bought, not in the best conditions if im honest but it starts, it can rev sometimes but not enough for it to move, it has a 70 kit fitted at some point and a doppler exhaust, 4.9g rollers, 21mm carb with size 86 main jet but im not sure what else, ive cleaned out the carb and jets and pit in some fresh oil. Could anyone please help me!!