A good clean inside and a 40W soldering iron with a screwdriver type tip and a red tie-wrap you should be able to weld it. I managed it on my ZX9R front mudguard which is notorious for cracking.
I do not always like what this bloke does but sometimes it is useful to see a technique tried out.
https://www.youtube.com/watch?v=gRCMIDILfEI