If the flywheel (red "disk" on top) doesn't move, then the innards are locked up.
I would pull the spark plugs and see if there's a mass of rust on them. If not, lay the engine so the spark plug holes are pointed upwards and spray some PB-Blaster penetrant down the holes. You can also use a 50-50 mixture of Acetone and automatic transmission fluid (ATF) for a very effective home-made penetrating fluid. Leave it sit for at least several days and then see if the flywheel moves.
Most folks have a favorite solution that they use to soak stuck engines, there might be something more effective than what I listed above, so we'll see what else works.
BTW, is the flywheel locked-up tight or does it rock back-and-forth a bit? If it rocks, that usually means the pistons may be stuck in the bores from sitting. A better chance that they'll free-up from soaking.
IIRC, the flywheel nut is 5/8" so you could put a socket wrench on it and try to move the crankshaft to-and-fro. Don't use tons of pressure on it in the clockwise direction, as with enough leverage you could break something.
You could also use a prying bar across the "teeth" of the flywheel.
On this model the recoil starter is mounted in the motor cover, and the starter pawls engage the teeth in the flywheel to crank over the engine.
If your red flywheel turns smoothly
(always turn it clockwise - never counter-clockwise)
Remove the spark plugs & lube as ed-mc suggests.
Once you have lubed the cylinders - rotating slowly by hand,...I put a socket on the flywheel's center nut, and spin the motor with a 1/2" drive power drill.
To test the spark, put the spark plugs into the spark plug wires, and rest the outside metal of the spark plugs, against the metal of the motor. Then spin the beast with the drill and watch for spark.
If no spark, you'll need to remove the flywheel, and see what condition the coils, etc are.
It looks to be an older style magneto, which is much cheaper to rebuild than most later solid state ones.
You have lots to do before hunting down a rewind assembly and top cowl (engine cover), but when you get to that point, post and we'll help you track down the missing pieces.
Check the condition of the lower unit's gear oil too.
If it has water in it or rust, you may want to throw in the towel & just do 'exploratory surgeries' on the beast.
My '65 3.9 still has the rewind in the top cover, so it must be a year or two later model. My '66 has the rewind mounted to the engine.