It would help if we knew how big the boiler is! What's it off – a Gauge 1 Mamod or a 15" gauge 4-6-2?
Assuming a standard model-engineering-type 3.1/2" or 5" gauge loco (or 1.1/2" or 2" scale traction engine) , 1/4" x 40 or 5/16" x 32 are likely sizes.
1/4" x 40 should easily open up too 9/32" x 40, which is a relatively common size.
5/16" x 32 (if there's enough material in the bush) could be re-tapped 3/8" x 32.. An expensive option would be 11/32" x 32 tap and die, which would be the least invasive option for a 5/16" thread, but it may take some searching to find them. (Amazon in the US has them, but neither Avon nor Tracy have them off the shelf.
Hope that helps.