If the alternator goes you can replace or rebuild.
Replace with another
When purchasing a second-hand unit you can check that the plug is still attached to the lead BUT due to the buried plug location it is impossible to simply replace the alternator. I ended up buying one without the plug and just spliced it in. I recall it being quite fiddly to align everything up and get it back into place – as usual patience and perseverance are the key traits.
Sep 2020: was quoted AU$165 for a second-hand unit from a local wrecker. I don’t recall my unit being that much, IMHO those numbers are probably spent rebuilding the old one.
Replace with ZRX
Apparently some US owners have used the ZRX1100 or 1200 units, obviously the same basic engine was used on later bikes (GPz1000RX) and it seems likely that the component was also shared. Probably work double-checking part numbers. At the end of the day newer is always better.
Here is some technical info from www.electronicamotos.com.ar for those considering rebuilding:
TECHNICAL ISSUE DETAILS
This Kawasaki model uses a Nippon-Denso car-type alternator that is mounted behind the cylinder block. Inside this alternator there is a field rotor with copper slip rings on which carbon brushes run. Around the rotor you will find the stator and in the front of the housing are the built in rectifier bridge and the electronic voltage regulator.
FAULT 1: OVERCHARGING THE BATTERY. This is usually caused by (1) a bad internal voltage regulator or (2) a bad electrical connection somewhere on the bike in the lead that provides battery voltage feedback to the internal voltage regulator (the voltage sensing wire) The replacement voltage regulator is our part# ESR025. This unit replaces the OEM unit even though it looks different. To find the bad connection to the voltage sensing wire you need to look at the leads coming from the alternator. There is one large diameter lead, usually RED, that is the output from the alternator to the battery. The thinner lead is the sensing wire for the regulator. Use a multimeter to measure the voltage on this lead to ground. It should be close to the battery voltage with the bike running (around 14Vdc). If you see a substantial difference (say: battery voltage 17Vdc, sensing lead 14Vdc)you should look for a bad connection somewhere between the battery and this sensing lead. Inspect all connections, the ignition switch, use spray contact cleaner on everything. Spend some time and you will probably find a bad connection somewhere. Fixing this connection should fix your overcharging issue.
FAULT 2: NO OR LOW BATTERY CHARGING. This can be caused by a bad internal voltage regulator inside the alternator, or by a bad rectifier module. A bad voltage regulator sometimes is caused by a shorted out field rotor that is drawing too much current from the voltage regulator. Inspect the rotor resistance between the slip rings which should be 3.3-4.0 Ohms. Another cause could be bad connections between the alternator output and the battery terminals. Check all connections and see if this fixes the issue.
Here is a link to a replacement alternator brush set.
Here is a link to a video on replacing the old rectifier