....... Compression? That would explain the rapidly increasing difficulty in starting before it finally would not start at all, wouldn't it? Or is there a rod, other than that stamped vertical arm, connecting to the throttle shaft?
Yes, I'm afraid it would. Are you saying that these symptoms developed suddenly and nothing seems to work to get the engine started? A rapid loss of compression is usually a carcked or broken piston....and I'm sure it would smoke badly.
Do you use your compression release everytimne to start the tractor?
Does the compression release fully close the exhaust valves when released? (Maybe one of the valves are hung up at the release mechanical arm?)
You might not want to hear this again but, have you filled the tank full of fuel, bled the cap screws sequentially and about 10 seconds at each screw (all the way to the banjo fitting on top of the pump)? It does take a good, long, sloppy and miserable 10 seconds starting with the bleed screw at your fuel shut-off valve starting with the left one (closest to the tank) then the right one....and finally the last on at the pump.
I know your thinking "This guy's nuts" but the slightest amount of air introduced into the system before the pump, and your tractor will NOT start.
Unless your engine overheated recently, it's not likely that a craked or broken piston is the problem. You may want to pull the valve cover and have someone turn the engine over while you watch the valve train operate. If everthing look OK then I would do a leak down and compression test.
Good luck and keep us posted.
Mark