Well, if you have "decent" (subjective term there) spark and fuel (that's not contaminated), the only thing left is compression. Do a compression check on the engine and see what the numbers come out at. If you are just trying to get it running but compression is low, one thing that I have done in the past with some LIMITED success (as long as it's a ring problem) is removing each spark plug and adding a small amount of oil (less than a teaspoon) in to the cylinder. This can help the rings seal and cause enough compression to fire. Now this is obviously something that you wouldn't want to do just for grins, but I have got stubborn low compression motors to run before by doing that.

Anyway lots of luck.

Cheers,
Firechicken