I went from the stock 35-amp alternator on my trusty Atomic Four engine to a whomping 120-amp alternator, but the new alternator was carefully specced by the Atomic Four guru Don Moyer so the only adjustment I had to make was a longer belt. I got the alternator for a song from another owner because it had been mistreated, but it was rebuilt quite inexpensively to good-as-new condition. Here's what I learned in this process that might be useful in your situation.
Several variables have to be considered when swapping alternators. One is the size of the alternator pulley, which, along with the size of the pulley on the engine, determines the speed at which it spins. This could be a particularly acute problem when alternators designed for large car or truck engines, that run at variable and sometimes high speeds, are put directly onto small boat engines, which often run at constant and low speeds. If those pulleys are mismatched to the new alternator's design, the alternator might not generate enough power, it might require too much power from the engine at lower speeds, or it might require an adjustment beyond the length of the belt or bracket to get proper tension. Putting too much stress on that bracket, or at the wrong angle, will break it. (In my case, Moyer had selected a large alternator that ran well with the stock engine pulley, and installed a proper alternator pulley on the new alternator that was a different size than the pulley on the stock alternator. The huge benefit to this design is that the old alternator can be kept as a spare and put back onto the engine without changing the engine pulley.)
On the plus side, it is pretty cheap to put new pulleys onto the alternator, and the engine if necessary, but the nuts holding them on sometimes require air tools to loosen. And of course alternator belts are usually abundant and inexpensive.
Alternator rebuild shops - the actual shops that do the work, not places that just accept old alternators and sell rebuilds - can very easily perform what we laypeople consider miracles. If I were you I would find a shop in the Bahamas that does this work and just have them rebuild your old alternator ASAP. It might even be worth shipping back to Florida, and if you're in a real pinch, the local place that did this or me and is intimately familiar with boat alternators is Bronx Ignition, 1424 Blondell Ave, Bronx NY (718) 792-2155. Do you have solar or wind power aboard? I'm pretty sure diesels don't require electricity to run, only to start, so you might be OK without any alternator at all for a few weeks if you're gunkholing.
A third consideration is a marine rating for ignition spark protection. This may not apply to diesels, but it is sacrosanct in gasoline installations that alternators have to have a special screen installed for spark protection, and off-the-shelf auto/truck alternators are not acceptable.