First, welcome to the forum.
Engines are like baking - get the ingredients right and it turns out perfect every time.
If you have compression, spark, spark at the right time, air, the right amount of fuel mixed with the air, it will run.
Now the spark timing is set by the crankshaft and as long as the flywheel key is good, then spark will be at the right time - so you say you have spark and compression - we don't know about fuel and air yet and when you say you have checked the timing, I presume you mean cam timing?
The only thing that concerns me is the manual tensioner - why a manual tensioner? Most people tent to over tighten the cam chain and that can prevent starting or make starting very difficult. The manual tensioner cannot be properly set without the engine running.
Here's my suggestion - back the tensioner off and run it in by finger just to the point you can feel it pushing on the chain, then back it off 1/4 turn - remove the carb and check the compression - should be at least 120 psi - if the compression is good, spray some starting fluid in the intake while cranking the engine - it should start and try to rev uncontrollably, but it will only run as long as you give it a shot of starting fluid - once you know the engine will run, you are down to fuel and air - service the carb and it should be good to go - if it won't run with starting fluid with the carb off, then it's mechanical (spark, spark timing, cam timing, valves, piston rings, piston to cylinder clearance............), but something perhaps off a little.
If someone cut the decompressor tab off the rocker arm, you might as well remove the spring and plunger from the head that activates the decompressor ans it will do nothing other than possibly cause noise at idle - if you want the decompressor to work, you will need to replace the rocker arm.
Let me know where you get with this.
Sorry for the late reply, but I run a motorcycle/ATV repair shop and have been busy with inventory and end of year accounting routines. I have an 05 TRX400EX and built a 460 (440 with a stroker crank) for a friend who races flat track.