I've not built one, but you can certainly use an Arduino to replace the 555 and monitor the battery voltage as well. For the diode, search Farnell/Newark/Digikey/Mouser/RS for the specification (>= 6A 100v fast recovery diode) and you will find lots of alternatives.
I've got something that should work in my digikey cart. (Picture attached)
What will I need parts wise to incorporate the Arduino for monitoring? I think i'll leave the 555 in place so I'll have a permanent setup that isn't dependent on an Arduino.