The clutch actuator is held in place with a threaded ring. Ideally, you would use a C-spanner to remove it, but access is so restricted that you will probably need to use a drift and hammer...

Watch out for the 3/8" ball under the clutch actuator.

Now you can undo the nut on the end of the mainshaft.

The nut on the end of the mainshaft may be extremely tight. Clamp the mainshaft firmly in a soft-jawed vice and use a power bar to undo it.

There should be a pin or screw to align the clutch actuator. You will need to remove it before you can get the mainshaft bearing out. If a screw has been used instead of a pin, an impact screwdriver might help to get it out.

Now you can drive out the mainshaft bearing. It is worth replacing this bearing as a matter of course as it is relatively cheap.