Generalities here, because time is different for each person.
First, go to kubotausa.com/parts. Shows for that machine, T1060-16800 starter assy. The solenoid, (switch, magnetic) Kubota T1060-16950.
Looking at Kubota part number, dealers want about $360 for one. That's the solenoid, not the starter assy.
The Hitachi number for starter is S114-833B. The Lester Catalog number is PMGR (permanent magnet gear reduction) 18288. You can find cheaper starters with these part numbers.
You could drill down to the solenoid itself. Internet shows these for sale as after-market.
Or, remove the solenoid and see what the problem is. The movable contact assy is a replaceable part should that be the problem. It may be that what is needed simply is cleaning and lubrication.
My personal preference, given the prices, would be to look at the solenoid first. If can't be fixed, then buy a new after-market solenoid. Or two. A total replacement starter is not going to have the original's quality.