An off the shelf module would probably be the best way to go. Most of these, unfortunately, tolerate only 12V in (your alternator could easily run higher than that). Some are specified higher (which you will need).
Much depends on the output of the rectified alternator (an alternator provides AC - your setup has at least a rectifier behind it). Most of these have no regulation, so it's perfectly possible to get large voltages under no load conditions.
Another issue you will face is if you suddenly disconnect the load (but not the regulator) at high current. You will experience what is known as 'load dump' and you can expect voltages to peak at up to 60V or so at the output of the alternator. (It's due to the sudden collapse of the magnetic fields from the output windings as the current goes to zero).
My suggestion would be to search for point of load converters (POL) - lots of people make them. - I used google search and got over a million hits although a lot of these are from trade rags extolling the virtues of the latest offerings.
Between your alternator and that converter (which you should spec at least at 20V in, preferably 25V) you should put a decent TVS (one of the SM8S family from Vishay or equivalent would do the trick -
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) to prevent your alternator from destroying the (quite expensive) power supply during load dump.
Cheers
PeteS