I thought I'd share what I've learnt rebuilding my S1 Eclat Beans gearbox speedo drive because there's scatterings of information here but I've not found anything like a guide so I hope this covers most aspects. My drive had the usual snapped drive cable/rod from the angle drive into the gearbox. Upon dismantling, in my case I think this was due to hard, dry grease seizing the gears. I'm aware others have different experiences.
To replace the rod I bought a 2nd hand MGC angle drive from Ebay. Others have cut down old speedo cables to re-fit but I didn't understand how the rod is held within the gear for one thing (soldered? crimped?) Admittedly I bought the MG drive thinking it was exactly the same but upon arrival realised the brass knurled nut is smaller. Otherwise it is identical. However, there are differing MG drives so examine photos carefully. There are 'new' MG drives easily available too. At the time of writing I paid £20 from eBay and it seems a 'new' MG drive can be found for about £45 rather than the £140+ asked for the Lotus version with the larger knurled nut. My plan was to remove the gear and rod fixed into it from the MG drive and fit into the Lotus drive. The rod length is exactly the same (but again, do your due diligence first).
The gear and rod are held in by the large Welch plug. Some plugs (my Lotus) have 3 tiny crimps: the MG unit plug was just pressed in. The crimps can be fairly easily knocked back with a chisel or sharp screwdriver. Applying a little heat to the casing will ease the plug. You shouldn't need much: I heated the MG drive casing until the oil within started oozing past it, indicating enough expansion to let it release. Turn the unit over and the gear shaft end can be tapped through which will push the Welch plug and the dished spacer behind it out. I found a socket that would just fit over the end of the gear shaft without pinching the brass ring around it. If you can't find a socket or similar, a drift, small punch or even a small screwdriver against the edge of the shaft will work; it is hardened steel so can take a bit of abuse -just try not to damage the brass ring -which can't. It should all come apart quite easily. With the Lotus drive I didn't even heat, (torch ran out of gas) but I had to be a bit rough.
The other gear and shaft which connects to the speedo cable was tight and stiff. Liberal carb cleaner and heating to melt the old dry grease within released this without having to dismantle but presumably, the small Welch plug on the unit can be removed in the same way and probably more easily if need be. The geared shaft inside the gearbox must be pulled and examined: mine was undamaged, supporting the theory that the angle drive seized as opposed to any other cause.
Re-assembly is, in the immortal words, "reverse of the dismantling procedure". Get as much oil into the drive as you can through the small filling screw: it will be impossible to get to this once fitted, which probably explains why the drive seizes after 40 years or so. The attached poor quality photo doesn't particularly help except to show the dismantled parts: on the left the rebuilt Lotus drive, on the right the remains of the MG drive, snapped rod and old gear shaft. With the socket there at top right I used to drift out the gear shaft, dished washer and Welch plug.