When mine broke a few years ago I got a spare from Bill Parker. He might still have some in stock. Cost about £100 and didn't come with any holes or fittings. Beautifully shiny though!!!
It is only a bit of wood though. Plenty for sale on ebay which is where I got a suitable lump of ash when the same thing happened on my Solo.
For the 21 I stuck the old one back together to make a pattern for all the holes and pulley cut-outs on the new one and took the opportunity to improve on the uphaul/downhaul/tillerpilot/extension arrangements so it might be worth thinking about all the features that didn't work so well on the old one!
Geoff P21/30 Cygnus