Is the motor still good? My snailbox was in far better condition than yours but the motor was just a heap of rust. If you do decide to 'upgrade' save the hamster wheel for me!.....
I fitted this set.....
http://www.hawkcaralarm.com/Universal4DoorRemoteCentralLockingKit.aspx
There's a bit of a 'write up' on how to fit it here.....
https://www.landyzone.co.uk/land-rover/defender-central-locking-fitting.317244/#post-4024986
A bit 'pricey' for what it is.....
https://www.ebay.ie/itm/Land-Rover-Series-Jaeger-Smiths-Gauge-Instrument-Lucas-E10-Warning-Bulb-Holder/273067667299?hash=item3f9419ef63:g:9XUAAOSw~T9agsxC
Oooh, you should introduce first but this might be what you're after. Regards.....
http://new.lrcat.com/#!/1235/674/675/150/1801
https://www.paddockspares.com/seal-43.html