Put a wireless mic on both the rabbi and the Bar Mitzvah boy and on the cantor as well if you can; a PZM on the lecturn is a possibility, but don’t rely on this alone as it can easily become covered with books, papers, etc.
Shotguns on stands in front of the bema if they’ll let you would be next best alternative.
If neither of these solutions are possible see if you can attach a wireless mic to the speaker of the PA system. I’ve even done this by attaching a mic to the tip of a C-stand and elevating it in front of a speaker.
You may be able to get a feed from the venue’s sound system, although this is often difficult and not very reliable.
The audio from a mic at the rear of the room — on your camera, for example — will not be very good, no matter how good the mic.
Be sure you scout the venue well in advance and talk with the rabbi or his/her representative. Also talk to the custodian if you can: you’ll be able to get the lowdown on power outlets, location of stairs to the balcony, where the rest rooms are, etc., as well as insights into where others have positioned their cameras.
We always take a step ladder to Bar Mitzvah shoots; the Torah will be carried among the congregation and shots from four or five feet off the floor reveal the entire activity.
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