I think it's fairly common, particularly when the existing charging system fails. There's a few methods folks have used to attach the alternator such as direct into the gear drive location, or adding a pulley on to the back of a hollowed-out generator or similar. For 6v tractors, it then opens up a range of other electrical options.

I refurbished my 6v generator and replaced the original regulator with an electronic regulator. It works perfectly charging at 7v between low and high idle. The highest amperage I've seen (generator is rated at 18A) is a bit under 10 briefly after a few pony cranking episodes. Now it barely registers as the battery (Optima 6v, a quality brand) hardly ever gets taxed. I wouldn't change mine to 12v as there's no point for me - it wouldn't provide any additional functionality that I want, would have taken longer to get set up, and did not cost any less than refurbishing my existing system

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