Sorry to hear that too.
Some pictures would help a lot (as you promised)
Without knowing anything specific about the O'Day, you might be able to get that section repaired by cutting it, adding an inner-sleeve, and then riveting a piece of new aluminium profile to replace the bottom 4 inches (or whatever). Several mast has been repaired like this, and some (older) are made that way from birth.
It also sounds like a great idea to replace all the chain plates.