If your starter is turning the engine over, but just not fast enough, it might be related to how cold it is. Is your tractor stored outside or in an unheated area? You could have a bearing binding in the motor or something else causing the starter to drag. Because you said you attempted a jumpstart and it didn't help, a dragging starter might be considered. Is there any way to warm up the tractor? I've used a kerosene heater to warm up a tractor. You could use a hair dryer, but that will take a lot of heating. The best thing is one of the turbo heaters if you have one available.
This is all just a guess and a stab in the dark. It's pretty hard to say what the problem is when you can't hear it or test it. /forums/images/graemlins/smirk.gif
Everyone has given you some good advice. This is just the only thing I can think to add considering your saying the engine is turning over but slowly.