The first step is finding the correct alt. There are two types of internally regulated alt. They are the SI series and the CS series. You are more likely to find an SI than a CS unit. The SI can be found remanufactured or at a salvage yard.
Parts are cheap and readily available.
The unit is rebuildable in the field. The CS unit is quite expensive and not rebuildable. The 10 has a 63 amp output and the 12 is 94. If your car is basically stock, the 10 will do fine. If you have a lot of power accessories, high perf.