Bell console 200 computer
 

Does the unit warn before the battery dies? I'd hate to lose my odometer reading.

Well if the odometer goes away when the battery dies, then it'll be likely to go away when you remove the battery to replace it.

Most electronics nowadays would store that in non-volatile memory. I'd expect that the Bell does too, but you can test it out right after you get it by removing the battery for a minute or even several hours, then put it back in and see. If it doesn't, return it and look for something else.

If I get a warning I'll write down the mileage and enter manually after replacing the battery.

If you don't trust the unit to remember your miles, it's probably safer to change the battery now as an experiment. If you want, you can put the original battery back in after the experiment.