The vendors sell the speedo faces, you can convert and be fairly close, but the odometer will still register in mph traveled. The euro and mph speedos have different rev/min with either 1210 or 1610 denoting the number of revs per mile the instrument uses.
I have the opposite as my euro GT5 was converted to mph, it required changing the ZF angle drive as well to get it to read close, so if you get a speedo you'll likely need the matching angle drive.
Other options are to have a speedometer shop make up a gear reduction converter. or change the internals to a GPS trigger.