To solve this you need to keep it simple and do one test at a time and assume nothing as you eliminate possible causes of the problem(s).
Do as already asked and put your meter at the battery and read what it does when the engine cranks, if you can get it to crank. 12.7V or whatever, means nothing until you put a load on the battery and then you get a reading of what the voltage drop is while cranking or when running or both.
Let's say you have a problem with your starter and or solenoid at the starter for illustration purposes. Once you crank the engine, with amps, (current) flowing from the battery, passing to the starter solenoid through the ignition switch to the energizing wire on the solenoid a circuit is completed. That is the start circuit initiated by turning the key at the ignition circuit.
By seeing what voltage drop is created by turning the key and reading the meter this will tell you if the battery has enough voltage to turn over the starter, or not. Your starter may not be bad, but the solenoid may be weak, or you may have worn brushes inside the starter, or whatever of a list of possible problems.
Ideally you would and certainly can remove your existing battery and have it load tested and check the specific gravity and electrolyte level of each cell in it to determine what it's condition is OUTSIDE of the tractor. If it fails any of the above mentioned tests it is time for a new battery. This may end up being the case- BUT it may not be your only problem.
Let's say the battery is toast. Get another one and install it and then continue to diagnose and rule out other problems like your starter/solenoid, etc.
Do not worry about 12.7 or whatever at the starter. Do worry about what the battery can put out under load. Post back results of above mentioned tests.
