4 things can be causing this no air, no spark, no fuel or no compression.

turning the engine over with a plug out of a cylinder which is not firing, but still attached to the lead to see if you have a spark
Get a compression gauge and test compression over all 4 cylinders one at a time, they should be very close.

Turn over engine without a plug in and you should see, smell and feel fuel

(although this won't tell you if it's the right amount of fuel).

The chances of you not getting air in are slim, so not worth checking at this point.

If you are missing spark then check dizzy and rotor arm as you've done leads and plugs.

If you are missing fuel then could need injectors replaced as it's isolated to 2 cylinders.

(2nd hand fuel rail should sort that for a few

££s's)
If you don't have compression then it's either the head leaking or piston rings worn, could be other causes, but these are the most common.

this is a big job to sort either way, although skimming the head and throwing on a new gasket is a lot cheaper and easier than stipping and rebuilding the bottom end.

Hop this helps you.
