I have a 2004 Kubota KX121-3 that I bought used with 2386 hrs on it and just busted a track today with 2506 hrs on the clock. The tracks on it are "Earth Quest" 350x52.5x88 but everywhere I look shows the KX121-3 using a 350x54.5x86 so I'm unsure if I have original or even properly matched up undercarriage parts or not. I read a good article explaining track measurements and measuring the track guides but it didn't go into how to measure rollers, idlers and sprockets to ensure all these parts match up for a specific track guide size. The rubber has been totally worn away on the track where the outer edge of the rollers have been riding on it so thinking the track guide on these tracks wasn't high enough for the rollers as I hadn't seen this on other used machines when I was shopping around. If I had to guess I'd say my sprockets are half worn, will using the existing sprocket, idler and rollers considerably shorten the life of a new track?
The manual will tell you what you need.... Here is a free download, I have got manuals from them before. https://www.manualslib.com/products/Kubota-Kx121-3a-8847657.html
You can get a parts breakdown and numbers off of Kubota's website by using the model or serial #'s. When buying aftermarket you can usually enter the model number and they will show which of their products would fit it. When I install new tracks I always put on new sprockets. That way its all new at the same time and I dont have to pull the tracks off halfway through their life just to change sprockets. If the rollers are tight with no play and the idlers dont have flat spots they should be fine.
You definitely have the wrong tracks on there, one other thing I can look into is if the 121-3 used different rollers for steel tracks and rubber tracks. If that's the case it could have been a factory steel track machine someone has swapped to rubber. If you think the rollers guides are too high and rubbing on the bottom of the track, that might be it.