I would suggest having a word with Peter Horton, he's the expert on speedos etc. Ask if if it's possible for you to send both the KPH and MPH speedos with the idea of making one good MPH out of both. I suspect Peter would just swap over the threaded part from the KPH speedo onto the broke MPH speedo and that's all you'll require, a quick service of the innards as well, maybe a refresh of the facia if needed and you'll have a brand new looking speedo good for some years.