One more thing to check, the Timing chain. If that sucker hasnt been replaced it probably has a mile of clearance right now. That can cause all sorts of problems and it can jump time. Not to mention the plastic breaks off the gear and can cause a clogged pickup tube. Is it backfiring out the exhaust or the carb?

Also when you crank it does it sound like R-R-R-REEER-R-R-REEER like it has a spot where it wants to spin faster for a moment? If it does that thats usually an indication that a valve is dropped. There is low ompression on that cylinder and the starter speeds up because of that.