There are not many instances in which I’d strongly recommend replacing with only an OEM v-belt...but this may be one. There are standardized and non-standard V-belt dimensions and if you install a belt that differs from what Kubota initially installed.... it might not reach the same years of service as you’ve already enjoyed. Also, rubber parts including V-belts have “shelf lives” in which they must be either chronologically renewed even if warehoused... or they must be specially packaged to preserve the integrity of the rubber compound. If you don’t know the dimensions of your belt when new then you may not be able to meet it’s dimensional requiements which will shorten it’s life. (If it’s as difficult as you say to replace it now.... you likely do not want to have to repeat it soon. See the following link, not to sell you from that provider, but to illustrate the differences in V-belts which otherwise might lead you to buy aftermarket and suffer early failure.)
Cross Section
www.mibelting.com