I'm sure Jerry at Clevedon Steam can help you out.
He has made all sorts of bits for me and I think he has these as a stock item.
I have used his servo gas valve as a steam regulator with great success
He also does a lever valve for servo operation that acts as a throttle or blower valve.
**LINK**
Edited By lnvisibleman on 09/04/2014 20:11:02
Edited By lnvisibleman on 09/04/2014 20:16:06