You'll need a bolt extractor to get the old piece out of the caliper and a replacement bleeder nipple - if you can't find a new one, you'll need a new caliper. I don't know what size extractor you need so take the piece with you to find the best size. You may need to drill it bigger and, if you do, then you'll need to disassemble the caliper to clean out all of the bits.
VERMONT AMERICAN Screw Extractor Set, Spiral Flute, 10 Pcs - Screw-Bolt Extractor Sets - 2LKP2|21829 - Grainger Industrial Supply
Sears, Lowes, and other tool places sell these.