To add a note of warning!
Original Chronometric speedos - and I presume, the replica ones - require a 2:1 drive when used in conjunction with 3.25/3.50 x 19" tyres. (It looks as if Roy has also fitted a Smiths speedo drive to his front wheel). If you have an original Douglas speedo drive, that has a different ratio (something like 29/10), so cannot easily be used to drive modern speedo heads.
Regards,
Eddie.