iIiiiiiiiiiiiiiiiYou could but you wont be the best results turning it by hand. If you remove the blade it will most likely not start so dont try. The blade acts as a flywheel, and trying to start a mower like yours with no blade fitted when you pull the rope you with get severe kickback and the rope handle will be snatched out your hand.....very painful.
If you have checked the cable is correctly attached to the operating lever on the gearbox, and you have cleaned out the pinions and lubed it all up, just reassemble it all, put the wheels back on, start it with the blade on and engage the drive if you can easily stall the drive to the rear wheels by holding it back from going forward, then stop the drive, adjust the cable up via the inline adjuster and keep doing it until the wheels continue to scrabble for traction whilst holding the mower stationary. If you can hold it back and the wheels stop rotating under load even though the cable is adjusted as far as it will go, its the gearbox........as i mentioned earlier, this is providing the belt is correctly tensioned, but if it isnt you wouldnt get any drive at all as the belt would simply slip on the engine pulley when started.