Drive side crank seals very rarely fail, unless there is wear in the main bearing. An impact gun of some sort is the best way of removing the clutch and crank pinion nuts.

A seal driver, made out of plastic bar, with OD slightly bigger than seal diameter, is the best way to fit a seal. Yamaha cranks are often fitted with o-rings, on the drive side, to seal the primary pinion spacer, and if yours has one it must be replaced.

Job completed, used a corded impact wrench to get the clutch bolt off. Might be my imagination but feels like wants to rev higher  and run better. Least it’s still working  anyway.  Many thanks to all for
the advice. 

Congratulations on a job well done!