I didn't install the speedo pinion and right angle drive before I fitted the engine & transmission and now it is a no go to do that task: not enough real estate!
Even if I did, I'm sure with my luck that I would have used the wrong pinion ratio anyway

So, when my ship comes in, Lotto happens and the stars align, I will go this way

Here is a link to the actual device which I believe is the same one: -
https://www.ebay.com.au/itm/314182969931

In the meantime, in between time, I am using a digital GPS speedo which is very satisfactory when driving: -

Low profile, programmable display, more accurate than something which doesn't work

Oh, and I don't think our authorities give a toss about mileage and stuff on historic cars, never seen or heard of any legalities in that area.