Well RR GP bikes run them down to 50°C (125F) but that is the compromise of wear vs cold as poss for optimum power at the point where you have a hint of detonation but you can stave it off with a constant low temp.
With pissy little radiators, low speed, dirt clogging them & heavy load of mud & sand that would be a foolhardy path to follow. I’d want to keep it below 70 (~160F) but I wouldn’t loose any sleep over it.
I would want to be sure that you don’t build in a restriction for worse case senarios (that killer long, deep sand, have to walk it up hillclimb)
PS Keep in mind that the temp measured at the head may be hotter than when it is measured at the top of the radiator.