By coincidence I'm also trying to clean up the top of a 50,000 mile FZS600 motor.
The drain hole continues in a straight line through the outer plug well to the inner.
This means that Plug 1 (4) needs to be out in order the clear the hole all the way to Plug 2 (3)… so debris and water from the inner plug well will drop into the now open outer plug hole.
I'd done a reasonable job of blowing all the loose muck out of the way before starting and now have plug 1 out. Despite the cleaning there is a lot of dirt that didn't shift and is poised around the hole. I plan to get the vacuum cleaner on it tomorrow to stop that gritty crap landing on top of the piston and then find a way to temporarily block the hole. I guess cutting the top off a spare 10mm bolt and then putting a slot in it will do the job.