It should have markings on it front or back. Springbok dealers bought speedos and etc from various vendors, so w out knowing whose it is, it'll be tough to find a match, if even possible.
Google vintage boat speedometers and click the Images tab. Might get luvky and find a match or similar.
Easier route would be to know diameter of speedo gauge and find a vintage speedo on ebay that fits hole, but may not be an exact match.
4 or 5yrs ago I heard of a guy that repaired/restored vintage gauges, car or boat didnt matter. But inexpensive they are not.... google vintage speedometer repair, I dont recall his name, how to reach him or where I read/heard it.
EDIT: googling vintage gauge repair produced a surprising # of hits