If you take the 160 Amp alternators for Yukon of Escalade and remove the diodes and regulator and manually change the field you can get them to put out 58V at around 80 amps...
The Ford Large Case 1G alternators work too but they make a lot of fan noise...
An ordinary garden variety 12v vehicle alternator needs to turn at about 2,000 rpm to charge a 12v battery.
The alternator pulley is usually about one third the diameter of the crank pulley, so as long as the engine idles above about 650 rpm the alternator light will go out, and the battery will be charging.
To charge a 48v battery the alternator will need to turn at four times the rpm or around 8,000 rpm which it will very easily do.
When fitted to a typical modern engine that might rev between 650 rpm to 6,500 rpm, the alternator will be whizzing aroud between 2,000 rom and 20,000 rpm which it will do very reliably. So running an alternator continuously at 8,000 rpm to charge a 48v battery is not a big deal.
Mechanically its not an issue, but it will require a 48v voltage regulator that you will probably have to make yourself. The field winding might require about four amps at perhaps 8v to 10v for full max rated output, and that will require a bit of thought.
This all becomes attractive because its relatively cheap to do, especially if you can score a free alternator, and figure out how to turn it at 8,000 rpm plus.
There are alternatives..... Some busses, trucks, and earth moving machinery often come with 24v electrical systems and larger alternators.
If you can find one of those, it should work fine at 48v needing only about 4,000 rpm, and once again a home brew regulator.
Its also possible to use a bog stock 12v alternator, rip out the diodes, and fit a voltage step up transformer to each phase. This will be a lot easier, and a lot more practical than rewinding the alternator itself. Its theoretically possible to run the original alternator at less than 2,000 rpm with suitable transformer ratio, but it will be limited in power output. While there will still be full rated amps availabe, less voltage and less rpm means reduced power. It may still be enough to be useful. It depends what you want to do.
Another totally different approach to charging a higher voltage battery (48v and above) would be to use a treadmill motor.
Here we have the exact opposite problem. A treadmill motor rated to run at 180v and 4,500 rpm and 12 amps can be run at one third the speed, so we then have 60 volts, 12 amps, and 1,500 rpm as a motor.
Used as a generator it will need to be run faster, perhaps at 2,000 rpm, as a guess, to charge at the full 12 amps and reach 60 volts.
That may be more attractive than a vehicle alternator for lighter duty work, and it has the advantage of not needing a field winding.
Many ways to skin a cat.
