1. The 175 is 175mm arm length. 170mm should work just fine and even better if your son is "sub adult" size. (I use 165mm because it's much easier on my old knees)
You won't find that model number new, and a used one may be worse than what you have.
You'll "most likely" have to replace the BB (Bottom Bracket) since newer cranks tend to use shorter spindles.
Just match up the tooth counts to the existing rings.

3. HOWEVER, rings have "shifting aids". These will appear to be broken teeth or cut outs or??
IF they are spaced symmetrically around the ring, expect them to just be that.
Look at a new bike and you'll see some type of them, assuming it's not a 1x.