how to replace a boss on a sovereign mower?

Hi

I have a petrol driven Sovereign mower with pully and boss there was a piece of old angle iron fencing sticking out of the ground the blade hit it and there was a bang and the engine stoped dead when I checked the blade it was beyond repair so I went to replace it and found that the two lugs were sheared off I need to know how to replace the boss on my mower thank you

Regards Jim.