how do I remould missing pieces of porcelain vases ect?

I have a pair of hand painted Noritake vases with broken necks, and the pieces are missing.Part of the necks are still there, and are 1.7/8ths in width.It seems that I will have to remodel the broken area of the neck from a mould to get a matching shape to the remaining part of the neck, and then repaint as necessary.What can I use for a mould to match the rest of the neck.someting that will accept a porcelain repair material such as milliput, without breaking during formation