Recently, I've heard of several people who's speedo gears in the tranny have failed. Mine included. I'm wondering if their age finally took its toll.
You might consider 4380r's idea. Speedhut allows you to customize the GPS speedo to look pretty similar to a real Smiths which is what he got. It's a little bit bigger than the standard Smiths speedo but that can be overcome as 4380r did.
If you want a Smiths one, they make an electronic speedo (SN5234-04CB on Classic Gauges) and you can get a GPS speed sensor (GPS01 on Accessories). They will also change the bezel to a round one from the 'half-V' on. It's a bit more expensive than the Speedhut rig, though. This is what I chose. First I got mine before I'd heard about the Speedhut unit and I wasn't sure I could fit it where the original speedo was.