We carry a full line of replacement undercarriage parts for the Kubota KX 121-3 series mini excavator. Some components of this undercarriage are interchangeable with other Kubota models for instance the top roller also fits the KX 161-3. The track is dedicated to this undercarriage system and is not interchangeable with any other track. See the image below and use your mouse to explore the part numbers and they should pop up with a link to the part number you need for your KX 121-3.
Here is the complete line of undercarriage parts for the Kubota KX 121-3 including the rubber track, Drive Sprocket, Bottom Rollers, Top Rollers, and Tension Idler.