It probibly is a problem with compression. If you are getting a spark then at least your crankshaft is turning which rules out the clutch not ingaging. The only other thing I could immagine in a broken rod or pin. Take off the head and see if the piston is moving up and down. If it is moving then it's probibly time to replace the piston rings and cylinder. Hope I could be of some help.
See ya,
Jon