[Barnsnake]

Some things to consider:
If you change to the 100 amp alternator you will need to make some other changes.
1. You should NOT run a 100 amp alternator through an original style ammeter. Ammeters are nostalgic, but just about every other aspect of them is a fire hazard. A voltmeter is passive and tells you what you need to know abut the charging system.
2. The wire from your current alternator-to-the-ammeter-to-the-battery cable may not be large enough to handle 100 amps. You may need to run a larger (or additional) wire to handle the additional amperage.

As noted previously, getting your current alternator checked/repaired would be the "easiest" solution to your problem.

Don't ignore the fact that hot-cranking problems are often due to starters, cables, and grounding. As a diagnostic, get it hot and in a location you can work on it. Verify that it cranks poorly or not at all. Let it cool and try it again. If it still cranks poorly, pull the alternator and get it checked.

Keep us posted on what you find.