The speedometer on these is cable driven. Does the speedo work fine when it is working?
If the odometer doesn't work even when speedometer does, I'd look at swapping the cluster. Plenty of people on the Volvo 300 facebook pages breaking these so you could pick one up readily enough
If you have a 6-gauge cluster (with external temp gauge and vacuum gauge) you can swap the extra gauges into a 4 gauge cluster by removing the blanking plates.