The battery will be nothing special, just need to identify it by part number, Jaycar or any battery supplier on the net should be able to help. The RTC battery could be in a holder on the main circuit board ( most likely) or a seperate package connected to the board. If it's a seperate package there is a mob in Sydney that can custom build a battery package to match, I think they are called Master or something like that.
Master Instruments at Milperra Sydney may be able to help, if the battery is soldered to the board then I suggest you get someone with electronic repair experience to replace it, you don't want to induce a voltage or over-heat the battery when soldering it on.
Its a euro 5 cummins. It doesn't affect the engine but it is logging an engine fault on the dash which is a problem because if there are other faults the driver wont know because the engine fault light is already on