If you have to store the machine outside try these options. Heavy Equipment Cover from Northern Tool. This fits NorTrac tractors from 20 to 30HP. So there is a good chance it would fit a Kubota BX tractors except for the backhoe. (If anyone has tried it on a BX please let us know.)
There are also some tractor covers on Amazon.com. These are mostly for lawn tractors - they may fit the smaller BX models.