@dr_kwak - if you read my question carefully i specifically asked if theres any other way of removing the drive - without the specified tool.
Thanks to everyone who replied with suggestions. I bought a strap wrench (wrapped it around the the main assembly) and managed to get it off.
FYI - for anyone who is thinking of removing the other three weights, there a bit of an improvement in acceleration. I might get 6 equivalent weights to balance everything out evenly.