Without any info on your 80 I will do the best I can to answer. The early 80's were cable driven speedometers, if the cable broke everything stopped. You have a speed reading so yours is electric and it is quite fragile but, the both the odometer and trip carriers for the cogs are also just as fragile, you might be able to glue them back again to keep them in mesh assuming this is the problem? I did envisage taking the assembly apart on mine to do exactly that but since a recent health issue my right hand is not super steady as there really is not a lot of space in there, I purchased a secondhand speedometer from a MUD member in the UK but due to customs problems here in Spain it has been returned to Ireland, once located I hope to see it very soon.
If you have patience, good eyes and a steady hand then give it a go, getting the speedometer head out form the binnacle is easy enough but clear a workspace and have plenty of light as well, good luck if you take it on.
Regards
Dave