Ok so after reading everyone's ideas, and thank you by the way to everyone that offered suggestions.
I decided to purchase a female 12MM coupler, with an OD of 20mm. I will then drill out the hole to 20 mm, insert the coupler. I will bevel the edge of the coupler so when it sits flush at the face I will have a nice grove to weld at the front.
In the back I'm pretty sure that the drill will remove some of the bracket in the back, if not I will drill holes and then rosette weld the back to lock it in further.
This should be a very straight forward fix... famous last words.
Here are my tools for the fix....
