Upgrade transmission or what can I do?

[jondee86]

Abuse kills gearboxes, not horsepower. Dumping the clutch, driving
over kerbs with power on, anything that puts sudden and high torque
loads on the gears. Helical/spur gears have only a few teeth in contact,
and the teeth can only sustain a certain load before they break off.

Sudden, high impact loads can break them. Repeated high torque loads
above design, will lead to fatigue cracking and breakage. Continuous
high torque loading with high temperatures or poor lubrication will lead
to surface fatigue, cracking, pitting and failure.

As said above, if you use the clutch with some consideration, and avoid
kerb jumping etc., the T50 will handle 200 hp from your high revving,
but relatively low torque engine Plan on servicing the box regularly,
and you should be fine.

Cheers... jondee86