How does it break needles? Right away, every time? Or does it sew a while, then break?
I'm a rookie, but bear with me. It has to be one of 3 things, right?
A) The needle hitting something
B) Something hitting the needle
C) The thread binding up an deflecting the needle enough to break it
I'd first handwheel it and see how it does. Then I might remove the needle and run it. Listen and watch for any odd behavior. Then maybe run it with a needle and no thread. If it doesn't break a needle, maybe add the lightest thread you can find and maybe it'll start breaking thread instead of needles. All that to try and isolate which end of the machine is causing the problem.
Have you checked to see if the feed dog has worked loose? Maybe it's that simple.
Good luck with it.