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Symptoms of this problem are: slipping out of second gear, especially under hard acceleration.
The solution is to have these gears fixed. The only issue is that it means tearing down the engine/trans assembly to its last nut and bolt. In my shop I charge about 35 hours of labor to do a project like this. Meaning that if you were to pay someone to do this job, the cost to repair would exceed the street value of the bike. In other words: the bike is Totaled.
Good news is that if the symptoms are minor the bike can continue to be ridden, often for many years after symptoms first occur. (depending on how hard you on are on the transmission, or how many miles a year you ride).