Ideas on a fix
 

Had a little oops on my radiator long ago and had to do late night garage trama surgey. I've replaced the radiator but if this can be fixed I'd like to keep it for a spare but being plastic in the corner I don't know if I could tap it and thread something in, so any ideas would be great-last plastic I tried to "tap" cracked so I'm leary of that.

http://i1025.photobucket.com/albums/...rd/oldrad2.jpg