[4-5-6-birdman]

First off when you say fouled I assume they are badly oil fouled to the point of not firing anymore? The compression test is the easiest and first test you should run. Check all eight cylinders and record the results, you should not see more than about 10-15% variance across the board. Post the results if you can.

Is the Bird consuming oil or having excessive blowby or pushing oil out anywhere? It is possible you just dealing with a flat worn out engine, if that's the case doing the head or valve seals will be a waste of time and effort.

There is one other diagnostic test that will tell more than the compression test and that is the leak down test. You can do a search for leak down on the web and it will show you the principal and how to make your own. It does require you have an air compressor however.

Probably at this point run the compression test and post the results and I can advise you from there. Good Luck