I would guess the noise is coming from the top end. If this is the case, you may be able to rebuild the heads/valvetrain and be done with it.
Have you run a compression check on all cylinders? It looks like it is misfiring. There is a good possibility a compression check will tell you what cylinder has the problem.
If you aren't really familiar with engines it would be well worth the expense of paying a good shop to diagnose the problem and go from there. Depending on where you are and what shop, expect to pay about $80 to $120 for an hour of diagnostics.