You will need to find a 1984 B-reg MG Montego EFi to get a digital dash. They were not optional but they were dicontinued after a few months and I am pretty sure no 1985 cars had them.
They are much rarer than Maestros with digital dashes because the VP version was not fitted with the digital dash and many more MG1600s and VPs were built during the 18-month period that they were available compared to Montego EFis.
The Montego digital dash is more sophisticated than the Maestro's as it has the following:
1) Electronic map of car
2) Service Interval indicator
3) Temperature sensors
4) Touch sensitive keypad (like Sinclair ZX81)
5) More complex voice synthesizer
To successfully fit a Montego digital dash to your car you will need to buy a complete Montego EFi if you are to get all the sensors around the car for things like blown bulbs etc.
However how about this for an easier option:
Get a Maestro digidash and fit it to your Monetgo. Loads of people have fitted this dash to their late Maestros with Montego fascias. Look elsewhere on this forum for Wonk_the_Sane's car and Jonathan Sellar's MG Maestro EFi. The only difficulty will be sourcing a speedo sender for your Honda gearbox which can be found on a Rover 820.
You may or may not want to fit the trip computer and voice synthesis unit. I have seen late cars with and without this. You will have trouble with this if you have a post '89 Montego (which I think you have as it is BRG) becaue you will have to replace the trim around the cloack and relocate your switches for the heeated rear screen, rear wiper and washer and fog light.