have done this for experiment in the 1980ties.
Mostly, I wanted to test how the alternator behaves if driven by a wind turbine.
(have also built a generator bicycle with a car alternator).
Then I found out that many guys in the USA have used alternators, driven by powerful motors at high speed (ca 3-5kW power and alternator speed ca 10000rpm) as arc welder generators. In this case the built-in voltage controller has to be taken out and replaced by something controlling for ca 40V of idling voltage and limiting the current if necessary. I guess that the internet is still full of these alternator welding generators.
The efficiency of the alternator is below 50% at 12V and when misused as a weld generator, even far lower.
In 1981 a friend of mine and I built a test stand for alternators to measure the efficiency in original and rewound state. We wrote a small book on rewinding the alternators for windpower. Still available (german language).
If interested, i can write down the experience with the alternators, or if you have a special project can give support. Also hints for rewinding the armature for different voltage/speed relations