There are no replicas of Smith's Chronometric speedometers that look anything like the originals and which retain the original case dimensions.
If you want the bike to look right then the only real route is to find a donor instrument and have it rebuilt to the correct spec. This is no longer a particularly cheap option....but still cheaper than the odd NOS WD type when they turn up.
I have mentioned this before, but another way to obtain a fully reconditioned WD speedo is to purchase a civvy chronometric of any type and contact one of the speedo restorers and do a swap with cash. They usually will not sell you one outright but in any case, it's a potentially very much cheaper way to go if you can find a "donor" instrument cheaply. Ferg