If the B6100 is like my B6000, it works on a siphon system. Coolant is warmed by the engine and expands, reducing its density. That makes it "float" on the cooler liquid and rise. The only route it can rise through takes it to the top of the radiator.
As it expands/floats into the radiator, it draws the cooler liquid up with it through the engine, where it warms, and continues the cycle. As the warm coolant goes through the radiator, it cools, which increases its density, making it sink the to the bottom of the radiator, where it is drawn up through the engine again.
The system works pretty well, is extremely simple, and has no moving parts to go bad. There is no thermostat.
The trick to making these work is to make sure the radiator is clean, inside and out. Especially in dusty environments, or, worse, places where dry mowing debris is blown up into the radiator or its screen, the radiator can't do its job effectively enough to continue the cycle and the system overheats. This happens in water pump systems too, obviously. The thermo-siphon system needs very free coolant flow through the radiator, since there isn't a pump to force it through.
Leaks in the coolant system, or low fluid levels, are fatal. While a system with pump will work even if low on fluid, these siphons require a full system all the time, or else the siphon is broken and coolant won't circulate. In other words, if your system is losing coolant, you need to fix it.
Some Yanmars have an identical system. They recommend a solution of not more than 30% antifreeze to 70% water, and apparently work better with less antifreeze and more water. Adding a surfactant such as Water-wetter is supposed to help significantly as well. On my B6000 I took a 1 gallon container of 50/50 coolant that I had half used. I filled the rest of the jug with distilled water, giving me a 25/75 mixture.
After flushing the radiator with a hose (You can't really run the home flush solutions, I don't think, since there isn't a pump to force the solvent through the system) I filled the system with my solution, added Water-Wetter, then topped off with distilled water. You want to mix the solutions as much as possible outside the cooling system, because the different densities of the liquids can potentially cause issues with the siphoning.
If you're worried about overheating, take the time now to pull the radiator out and take it to a shop. Have them rod it out and scour the inside passages. Clean the outside of the radiator well and straighten any bent fins. Make sure your cooling fan is in good shape, with no chips or broken blades, and the shroud is snug fitting and in good condition as well. Make sure to clean the screen before every use, and regularly rinse out the radiator core.
Some Japanese tractors I have seen had the neatest warning for overheating: They put a little whistle in the overflow line. When steam is produced, you hear a whistle. That may be an easier find than trying to plumb and wire a temperature gauge. You also may be able to route the overflow so it runs onto the exhaust, giving a cloud if it gets hot. That's what I did on my KTM dirtbike.
