I presume you are intending to use a "frost plug" type. "Frost plug" is a bit of a misnomer, the plugs are actually used in the block-casting process, and if your cooling system was full of water and you freeze-thaw it a few times, the block will still crack. Back in the really old days of thick block castings, those plugs could push-out to relieve frost pressure, but for the most part, thats a thing of the past. But, we still call them "frost plugs". Naming a casting plug a frost plug does not change your block heater install.
I'm guessing 30mm, thats for the drive-in type. Bolting an o-ring sealed block heater into a hole in the block is a hay-wire way to build something but it probably won't fail during the warranty period. Auto manufacturers who are famous for cutting-out pennies from the manufacturing process would never spend pennies threading a hole in the block that doesn't need to be threaded. Kubota engineers (who seem to like to do stuff better) have decided that if a block heater is to be inserted into that hole in the block, that hole is to be threaded, and the threads will be assembled with sealant, and it won't leak for a very very long time.
So your first step is to determine if the hole in the block into which you intend to put the block heater is threaded, or is un-threaded with a drive-in plug. If your unit was built-intended for a warm climate, or is old enough, all of the casting holes in the block may be un-threaded. If un-threaded, it will probably be 30mm (1.1811"). If it is threaded, it will probably be 3/4" npt - 3/4" pipe has a nominal OD of 1".
When sourcing a thread-in type block heater, some block heaters may be designed for an engine coolant jacket that has more room than yours. A V2203 4 cylinder engine may have a roomier coolant jacket than your 1-litre 3cyl. So be prepared that the "Kubota" block heater you can buy on Amazon might not thread into your block.
