Did these saws go swiming? How long have they been sitting? What brand and model? I've got some project saws that have been sitting for years and still turn over just fine. As far as using BPB, I wouldn't. BPB is some strong stuff. You may not want to use it since if it gets into the crank it could (I'm just speculating here) do harm to the seals.
I would do as previously mentioned, pull the muff. and check what it looks like inside. Pulling the jug is no big deal and makes cleaning up piston and jug walls easier. If piston and walls o.k. then you're gonna have to tear it down since bottom end and crank bearings may be at fault.
Curious as to what you find.
Later,