Could you get the old one welded? (Not ideal, but should get you back on the water). You'll need to get the screws unseized first of course - welding in situ would be a bad idea.
In the U.S. there is a product called
PB Blaster that works well at unseizing things. My local rigging guy sprays it onto seized fittings over a 2 week period, occasionally hitting the head of the fitting with an impact driver.
If I hear of a tang available locally I'll send you a PM.
Chris.