Greetings, I would be a little nervous about using the valve specified by Jason, because it lacks a gland on the operating spindle, and whilst rated for 150 psi, at the elevated temperatures found inside a boiler or smokebox, i have found the, I think, PTFE, seals soften and start to leak. ball valves are sold for higher pressure & temperature operation, and they do have a spindle gland. They cost abut 3 times the low pressure ones.
I fitted a cheaper one to a 71/4 Britannia, and it blew after a couple of steaming, replaced with the higher pressure version, it now has 6 years concentrated running, no problems.
The use of a ball valve in this application transforms the controllability of a large locomotive, with finger tip response. Some regulator designs can be all or nothing.
Regards
Brian