Here's a how-to on the knock sensors, they run about $100
http://www.subaru-svx.net/forum/show...or+replacement
I'm guessing the leak is the valve cover gaskets, you would probably not be able to see a head gasket leak (and if you did, you would have much bigger problems). Is the leak you describe oil dripping from the bolts you can see looking down at the drivers side of the engine below the intake manifold? Those are the valve covers, unless your loosing lots of oil, don't worry about it. You might be able to tighten the bolts slightly, but svx_commuter told me they were a special kind of bolt, and usually will not tighten.