The answer is "it depends". We've tossed a whole boiler after 2 years due to excessive corrosion. So, it may be possible to use brass fittings _IF_ the owner is willing to scrap it after a year or possibly 2. I've also had brass fittings from Stuart Turner go zipping past my nose off a 504 boiler. (the safety valve, strangely enough, and no, I wasn't busy clouting it with a hammer at the time...).
The boiler we scrapped was electric fired, made from some 5" copper tube (or I think it was 5", it might have been 4" ), with steel end plates with O rings. The element was a 1500w electric element from a small hot water tank. The 1/2" plate ends had severe electrolysis corrosion after about 1 1/2 years, to the point of being unsafe for further use. (more than 1/2 way pitted in isolated locations). We figure there were a lot of impressed currents in the boiler in relation to normal boiler designs, because of it being electrically fired and with more copper than steel. (both are not good for boilers).
The good news is that it was just the end plates that were scrap, and the shell was OK, with fittings getting stripped off. I think the 504 mentioned ^ was the direct replacement, which was a step backwards as it required either gas or large quantities of alcohol to fire. (I got the short end, and had to use Alcohol, as dad had the propane @ his house).
Good places to start with for any boiler design are doing the math involved. Model Boilers & Boilermaking (KN Harris) would be my go to book for figuring strengths. If it is a commercial design, someone _should_ have done said calculations, but it is always reassuring to do them for yourself. (also puts that school learning to use !). A boiler that won't steam your model is a disappointment. A boiler that isn't strong enough is a bomb. (the 970 BTU/LB of latent heat is a LOT of potential energy).