If you want my advice... install the mod you mentioned in your first post.
It does work with 2.0.21 however it may need some tweaks or you need to alter stuff when adding it to a 2.0.21 forum but it does work, I have installed it on 3 2.0.21 forums and it works fine.
However if you do install it then I would also suggest you install the 'large group' fix by the same author as trying to view group permissions with groups containing more than about 3000 users will not give you what your expecting.. but with the fix it works fine.
It's not the years honey, it's the mileage...
I do not provide any install services for phpBB, Mods or Styles.
Please do not pm me for support/scripting help - you won't get any reply. If you have a question then make a post in the appropriate forum.