Hi Mike,
Two other items I posted about on these forums when I was considering Northumbrian were lack of pix or drawings of the foundation ring area of the boiler, and no spec whatsoever for springs on the safety valves. Just mentioning it as you will probably need info on these areas if you build the engine.
For the safety valves, a while back Gordon Smith published several generic designs of “soft acting” valves for different scales in EIM magazine. They work really well. If need be you could Google these valve designs and use one of them for your engine.
JD