From memory, every Smith Tacho I've taken apart has had a Pot in them for Calibration. I'm unsure if it would have sufficient range to take it from 3 pulses per rev to 2 pulses.
4 Cylinder types in that style, I would think would be much easier to come by than 6 cylinder ones. Maybe get in touch with a MG Group (and what other cars may have used these) and see if you can swap it for a 4 Cylinder type. Personally, I like modifications, but I like them to look finished and manufactured. I note on the face of this gauge, it says '6 Cyl', that would bug me every time I got in the car. I'd suggest this as a first option.
If you do want to go down the road of re-calibration, even if there's not a pot or not enough range in the pot, it would still be possible, there's 'pulse changers' available for this or it would be possible to change a couple of internal components to get the outcome you're looking for.
As a final check / calibration of it, find a Mains Power to 12 V AC Power Pack (or just a small transformer will do if you can look after the safety aspects). Note that most Power Packs are DC. That's no good here, it must be AC, though a DC one as well would be handy. Connect the Tacho up to power it up, then connect the AC Power Pack between Earth and the Points connection of the Tacho. As you live in the UK, adjust the Tacho until it reads 1500 RPM.
