To increase your chances of finding a speedo, you can probably get any chronometric of the correct basic type and then get it calibrated and a new dial with correct number put on it by most specialist repairers.
Variants to look out for when buying one include the bezel type, presence of trip and light (rim type bezel for nacelle, but bezels of various types are available) position of trip cable etc, but a specialist may be able to take pretty much whatever you give him and “convert” it (or exchange it), converting a side to bottom trip cable (or vice versa, cant recall) might require a few hard to find parts though.
Just a thought....