"if I top off this dead battery with distilled water, will it charge back up and hold charge effectively?" - depends on "effectively" means to you. It will be very poor compared to a good battery. Amp hours will be drastically reduced. It will never reach full charge SG. I wouldn't recommend using it long term, but try it and if it holds enough charge for you... OK.
On highway rides it may be acceptable but I think in city traffic you'll find the lights going dim and the horn feeble as the voltage drops. If you have electronic ignition it may shut down, standard ignition keeps sparking (weaker sparks though) until a rather low voltage. Electric starting will probably be just a memory.
You can get a maintenance free AGM battery that will fit. Carefully (!!!) check terminal polarity before connecting any new battery, they come with positive and negative either way around - you must not connect a battery backwards as that will fry your rectifier instantly and maybe melt some wires or start a fire in the wiring.
The battery shouldn't go dry all on its own unless left for several years. The vent is very small and evaporation glacial. Usually dried out batteries indicate overcharging... but that's unusual on a 400F if the headlight is on all the time. You should check the float voltage at least, with a fully charged battery the voltage should not be above 14.5V with the engine over 3000 RPM. I wire a voltmeter to the battery and then duct tape the meter to the tank where I can see it when riding, than go for a highway run: that eventually charges the battery to where the system voltage is controlled by the regulator. It's possible for the regulator black wire voltage to be a few volts less than the actual battery voltage because of dirty/failed switches and wiring harness connectors; that makes the regulator want the battery voltage to be those few volts too high. With headlight off, the battery can "boil dry" this way - overcharging turns the water into hydrogen and oxygen bubbles. With headlight on the load is generally too much for the weakened alternator (reducing the regulator voltage reduces the field coil power which reduces alternator output) can't overcharge the battery.
