I replaced my alternator, and my voltage regulator. I had the same issue. If the battery is smoking from the vents, it is getting a charge, so it probably is just your voltage regulator. I had the exact same issue this past week on my G154, and it was the voltage regulator. You can test it by using an alternator tester, or volt meter on your battery terminals while the tractor is running. If it is charing above 14.3 volts, then it is probably your voltage regulator. The voltage regulator is on the back side of the dash. If you remove the battery, you can see it by looking under the dash from where the battery sits. It is a square metal box. I just bought one because the wires on mine were brittle, and broken, and the new one fixed the probelem imediatly. It charges at 14volts now, and before I replaced it, it was charging above 16volts. If the regulator is bad, it will just keep charging the battery higher, and higher, overcharging your battery, and that will ruin you battery. Make sure that you replace the lost fluid with distilled water. From the sounds of it, I think your regulator is probably bad, and not the alternator, because when my alternator went, I was not getting any charge at all at the battery. When the voltage regulator went, the battery was overcharging, and boiling over from the vents. This is the voltage regulator that you need:
8003 90 New Electronic External Regulator for Mitsubishi 12V | eBay.
This is from an older post this year about the alternator, but ILIKEURTRACTOR is right about rebuilding the alternator, it is cheaper:
It's a Nippondenso alternator, and an exact match for the OEM alternator. I did quite a bit of searching when I bought it. It is exactly the same one that came on the tractor when it was new. It is not cheap, it is $149. It was a big job too, because the screws that hold the alternator are on the inside of the fan belt area, so you have to do a lot of reaching. Also be very careful with the plastic plug, that plugs into it, because they can become brittle when the are old. It was very nice to have the new alternator in it. The tractor seems to run better, and the lights are much brighter, plus I don't have to jump start it all the time.
This is the link for the alternator, you also may be able to search the model and find it cheaper, however make sure that it matchs perfectly, otherwise the plug may not fit. :
New Nippondenso 12 Volt 35 Amp Satoh Alternator: Alternators & Alternator