I see much twaddle about crank loads caused by a 300cc conversion.

The 158cc forged Wiseco piston/pin weighs 105g, the standard cast
124.4cc piston/pin weighs 110g. There is no more load placed on the
crank because of the pistons. It's actually less. Fact.

However, the conversion adds approx 20% more torque. It is this that
adds more stress to the crank. If we assume that the safe working RPM
for the standard motor is 12500rpm, then we can work out the new safe
working rpm for the kitted motor. This works out to be 11400rpm (the
calcs are too complex to show here).

Since the peak torque of both standard and kitted motors is typically
around 10500rpm, then you should be safe by not revving beyond
11000rpm under heavy load (high speeds) for long periods.