The sensors are notorious for getting jammed in the hubs (i.e. rusted in, even though they're plastic). They can't be repaired once bent or split, so you'll have to get a new one. Make sure you get the right type, as later Multiplas use a different type to the early ones.
If it's broken off, get a metal drift, or the shank of a broken drill of the right diameter (about 5 or 6mm) will do and use it to tap the broken bit of sensor out of the hub with a hammer. Before you put the new one in, use a rolled-up but of wet-and-dry paper to clean up the hole for it in the hub. Rub a bit of silicon lube (like the stuff plumbers use for plastic pipes) on the body of the new sensor before you fit it. This will help to stop water getting in, so make it less likely that the new one will rust up. Don't use oil or grease as it can make the plastic body of the sensor swell up, which is just as bad as having the thing get rusted in.
You'll probably find that the new sensor is going to cost you as much as the rest of the original job you took the hub off for......
