I made a two part silicone mould of the original mouth template. After experimenting with different mixes of fast cast and iron filings, I realised that the magnet that I had cast in the head, to hold the mouths on, wasn't strong enough. So I bought some strongwer magnets, replaced the existing one (under th nose) and gluesd in 2 more under each cheek to hold the mouths more securely.
After casting a batch of mouths I sanded them, filled any holes with superfine milliput, used a dremel drill to cut out the shapes for the open mouths, resanded, sculpted milliput lips and also added milluput under the chine to extend the jaw for the open mouths. I had to complete all of the mouths before I could paint them or the head so I could do them in one go, to be sure they were the exact same colour.