The starter motors are very reliable. The decision to carry a spare motor depends on too many factors and personal preference. In general, carrying around a spare motor is not necessary. However, if you are traveling to very remote destinations and the engine is getting up there in age that can certainly be considered a reasonable precaution. Kubota parts are widely available worldwide and we support them with our parts store as well.
Aftermarket starters are another issue. We do offer a few aftermarket starters as an alternative but there are so many brands that are now available. The problem is some of these starters may "fit" but not have the correct windings and torque necessary for the Beta application. We recommend the OEM starters as they are designed and built to the correct specifications.