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.
Try to make a living, not a killing. -- Utah Phillips
Don't believe everything you know. -- Bumper sticker
Everybody is ignorant, only on different subjects. -- Will Rogers
Law of Logical Argument - Anything is possible if you don't know what you are talking about.
If at first you don't succeed, try, try, again. Then give up. There's no point in being a damn fool about it. -- W.C. Fields