Unless you'd want to hack up shells for "tinplate" locomotives, usually one piece
castings, I'd use (and this would be more expensive, possibly) the individual parts
that are available in O scale, such as bare boilers, steam chests, appliances, cabs, all available individually IF you can find them...in an O scale specific shop or show, and on the net. I don't think the Strasburg O scale show is on either side of York week,
but that can be a place to find stuff like that if you attend York.