Before you begin your fast and afterwards in the evening be sure you drink enough water to hydrate yourself sufficiently. Reduced saliva and a dry mouth make your breath odour stronger.
In addition, practice regular oral hygiene which includes brushing your teeth, flossing between your teeth to remove plaque which will harden into tartar; and, scraping (or ...
The thing that serves as the major reservoir for bad breath (or the bacteria that cause it) is the tongue.
Brush your tongue, and you should have better breath. This doesn’t resolve all sources of bad breath, of course, but it does resolve one large but easily-tackled source.
The 70% Ethanol gel-ified hand-disinfecting liquids that we normally use against SARS-CoV-2 do a pretty good job when applied to the armpits (and their immediate surroundings) in a sufficient quantity, for a sufficient amount of minutes.
I mean, it seems to be a solid win for a few days, and the positive effect (even if fading) can last almost into a week.