Used to use a cycle speedo from Halfraud's, with the magnet attached to the hub and the pick up on the brake stay. Worked well. At the last rebuild wanted something similar, which also displayed RPM. So with Electrex ignition, I fitted an 'Acewell 1550" combined electronic speedo and rev counter from Acewell.co.uk near Newcastle. The speedo uses the same magnetic pick-up as the Halfraud's cycle speedo, whilst the tacho picked up it's signal from either the trigger wire to the CDI or attached directly to the CDI. Inspite of wrapping the pick-up wire various number of time around the signal wire to the CDI, or directly wiring to the CDI, as per the clear written instructions, I was never able to obtain an accurate, steady, RPM reading and now after 20K miles the readout has failed!
So, I have acquired a Smith's mechanical 4:1 ratio tacho, produced for the 1965 Cotton Telstar that when connected up indicates a good steady reading. As I also own SRX 600's, I have borrowed a tacho from one of them, as they are 4:1 ratio instruments as well and using the OME tacho drive cable this too has performed well, if a wee bit sticky at the bottom end ( I suspect due to a lack of use!).
Both instruments are of similar, if not the same size as the havering needle Smith's instruments fitted by Ducati, but I haven't trial fitted them into the OME instrument binnacle. I suspect the vibration might harm them, so would use their original mountings or construct a holder that is fixed to the bike using small diameter chainsaw handle AV bushes, readily available either online, or your local garden machinery dealer.
If you want detailed dimensions of either mechanical tacho, I shall try to oblige.
Good health, BillR