If your battery is going dead with the negative cable disconnected, then the battery has an internal short circuit (bad cell) and it is no good. Being that it is only a few months old, then take it back to where you purchased it and have them test it and replace it. Hooking up the cables incorrectly will not hurt the battery.
Moving on to the dynamo, if it is the cause of the parasitic drain, then just replace it with a used dynamo. If you want to install an alternator on the tractor, check with a local alternator shop to see who makes a quality alternator that will fit your tractor, and make sure that the alternator manufacturer offers service parts. My Lister Petter diesel engine came with an alternator that is not serviceable. No parts were available, so when the internal regulator went bad, the alternator became a junk part. I did find an internal regulator part on eBay that fit into it, and replaced the old regulator with the "new" replacement. That one lasted about 2 hours and then it stopped working. More diagnostics and I learned that the OEM cable from the alternator to the battery terminal was of insufficient size for the output of the alternator. Put a heavier cable onto the alternator to the battery, replace the internal regulator again, and it failed again. Now I keep a battery maintainer on the battery and the engine starts every time. The Lister Petter engine drives my 16KW generator head. No need for an alternator on a diesel engine generator. Been using the battery maintainer this way for the last 8 or 10 years.
