Already done that one.....
Mazda now sold and replaced with a BMW Z4 in Sterling Grey.....but it has red leather seats.
Anyway, the speedo will give problems and there are a number of causes, plastic wheel in the gearbox, cable, angle drive, odometer drive in the head all can cause the symptoms you describe. The clock also will give problems, your's is the early type and that can be cleaned by any good clock maker but it will give problems again in a very short time.
The clock can be removed with the dash in situ which leaves the car still useable. I may have a spare speedo head you can borrow if you don't get one. The twin dial speedo head is the same I believe.
I think I said before that the sender can get covered in corrosion in the manifold, insulating it a bit and causing false readings.
Editor of the VVOC Magazine ReVival
"When you have a Firenza like mine, who needs any Minis?" copyright David Maxwell 1979.