OK, I know nothing about the Samurai speedo set-up, sorry, I'm assuming it's an electronic one.
As a variation on CR's comment about bicycle speedos, how about a British motorcycle speedo drive?
My '55 AJS uses a Smiths speedo, driven by a self-contained gear drive clamped to the rear axle. That's a pretty standard set-up for bikes of that period.
I don't know if it would be possible to adapt such a gear drive to your car, or if the ratio is suitable.
If that doesn't work, how about an electronic speedo with the Smiths dial swapped in?