There's about a gazillion "bronze" alloys, so it's hard to say for sure. In machinability, bronze (depending on the alloy) runs the gamut from not too bad to difficult. You'll just have to try the castings to see what you have. Presumably, whoever made the castings had some interest in making them machinable.
*Sharp* tools are always good, especially so with bronze. I haven't tried carbide on bronze, but I'd guess that you'll do better with hss.
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