Sorry to hear that... double whammy there..
Is the mast in a general welding shop, or a rigger's shop? This type of repair should be sleeved rather than simply welded back together.
You'll need to compare all dimensions or you'll be into sail and rigging modifications as well. You should also determine why your rig failed... a shroud or stay must have parted or failed.
Ultimately you'd be better off with a new mast to the original specs.. I'm assuming that you're not having your insurer fix this for you....
1984 Fast/Nicholson 345 "FastForward"
".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)