The dreaded exploded boss....I had one totalled and a couple cracked that would fail in no time. Best to break all the cracked and broken bits off.
Using some old tubing that's about 10mm ID, I cut and made a mould around the boss.
Using a bamboo skewer and some electrical tape, I made a shaft to go into the centre of the boss. In hindsight, I would put tape all the way up the skewer to make pulling the skewer out easier.
The man at Bursons reckons this stuff is alright, but I guess any similar product will work. You drop a bit of adhesive in then some of the powder and build up inside the mold to the level of where the boss once was. This stuff can be sanded after if you go too high.
After a couple of minutes the stuff has gone off and is hard. Cut the mold off and trim the skewer back (if I had of put tape further up the skewer I could have pulled it out, making the next step a bit easier)
Then carefully drill out the skewer using a 3mm drill. Don't go too deep. Once that's done, grab a 4.5mm (or whatever you need to suit the screw that's going in) and gently drill the final hole. When you see red tape, you know you're near the bottom.
Be careful screwing the screw in, start the thread then go in quarter of a turn, back it off, another quarter, back off.....until the screw is in far enough. The stuff is fairly brittle, I recracked one of the bosses and had to rebuild it again.
Good luck.