You sure it never went ... kapow , kachink or kachunk ? (translated as ... snap , crackle and POP) ...
What's it actually doing ? ... Missing gears ? Jumping out of gears ? Crunching ? ... You have a good basis for a rebuild with the aim of improving it ... Then change whatever you were doing with it before it failed ... No point going to a different type of engine only to conjure up a repeat performance ...
In most cases , tranny failures are the end result of how the rider treats it ... ie too long between oil changes , use of too thin an oil (remember ... these engines are a GEARBOX as well as a motor ... if they wouldn't put thin engine oil in their cars gearbox or diff then what makes people think a bikes tranny will live a decent life with thin engine oil ) , improper use of clutch , not synchronising shifting with throttling , shifting under load , revving the engine and dumping the clutch to make the rear wheel spin on a regular basis ... (if it won't spin with the clutch out then don't try to force it ... errr .... that is ... unless you're good at rebuilding engines , have a thick wallet ... and don't stress out when it loses weight rapidly against your will) ...