Hadn't noticed this before, but for what it;s worth I had a vaguely similar problem with the Merc. It's of an age when the rectifier was just that - an AC-DC converter. No regulation.
Only when I got a Voltmeter in the form of a display option on my "new" plotter, and nearly broke my jaw as it dropped - at 1500rpm the alternator was throwing out 19.7V, and it kept increasing from there..... (at least it explained my battery's health!
So, Having looksed around I finally settled on replacing it with a Reg / rect off a Honda 50Hp 4- stroke. (a combination of partly cost, but mostly availability!). If you have the crimp tools to change the plugs / terminals on the ends of the flyleads you could in theory use any suitably rated (i.e same or bigger amps capability than your alternator) reg/ rect unit off any outboard. )
The pain will be wiring it in, biggest PITA is mounting it. (but in your situation it could live by the battery & the wires from the engine "feed" AC - if you don't have an electric start, you donlt need a +12 gnd.)
If I get time tonight I'll post a pic of my lash...errr....I mean set-up!
(Which does work well, all joking aside)