I have an 80 hp Lehman and have looked at the serpentine belt system. The PO put in a 100A Balmer alternator and the ARS-5 external regulator. When looking through info on the ARS-5, I came across a video on what it can do and the fact that most people simply install it and never change the factory settings. It can be set to battery type, including the manufacturer's recommended bulk, float, etc. settings, as well as tailored to your boat usage.
One of the settings is for a "Belt Load Manager." Standard 1/2" V belts are supposedly limited to 100A alternators, but with the Balmer Belt Load Manager setting, some have gone with larger alternators using a single belt without issues. Here is a past TF post using a
1/2" belt on a 120A alternator.
If you are looking for a project, a serpentine belt conversion might be a good one. $500 and 10 hours of labor if all goes smoothly. If you are looking to reliably run a 100-120A alternator with your standard belt, the Balmer exterior regulator cost 1/2 as much and 1/10th the install time. Where's the fun in that?