Hello Wotwot

It probably would have been easier doing this with clutch and engine
in situ. Relevant instructions are here.

https://www.outdoorking-forum.com.au/forum/ubbthreads.php/topics/58163/MODEL_45_-_Edger_Fitting_Instr.html

An old chain wrapped around the sprocket (to protect it) and a plumber's wrench
over that should lock the shaft so that a bar can be inserted to unscrew
the boss. It's a regular RH thread.

Alternatively, you could refit the reel sprocket and chain and lock the
reel with a wooden hammer handle or block of wood. Then unscrew the boss.

Hope this helps.
---------------------------
Jack