About how many miles does this engine have on it since the last rebuild? If it's been properly maintained with regular oil changes, etc. you should still have a lot of miles left in it.

To track down the source of your knock, try a mechanic's stethoscope. I thnk I bought my last one for $4 at Harbor Freight. If you can't find a cheao one, use a short length of garden hose, or even a stick that you can rest against your mastoid bone behind your ear.

It should allow you to localize the noise to a specific area, if not right to a defective part.

With 5 main bearings, I'd agree with JohnMc; it is most likely a cam knock - although with worn rocker faces, you can't adjust those properly using a blade-type feeler gauge.