I tried to find a fan/alternator belt for my tractor and found out the dimensions are all metric. I could find a very close width and length in inches belt, which would work, but the angle of the belt is also a metric measurement and an inches style belt would not mate well with the pulleys. Maybe if I tried for a Toyota, Nissan, Honda, etc. belt, those would work but not worth my time, so I just ordered from a Kioti dealer. I believe this is the one for your tractor. T4345-72102 Belt Air Conditioner, $22.30
See this link from Michigan Iron and Equip site. Error
Kioti DK50SE hst w/hoe & John Deere 3720 hst cab & John Deere 3120 hst w/375 hoe
I went to dealer and they had it in stock $23.91. Old belt looked good just stretched and no more room for adjustment. I have not installed new one yet and it is loose but working. Both belts had A58.5 on them.