The junction box should be behind a cover, and even if that's missing you can't spray it directly with the wheel on. There is a 2-pin connector clipped to the side of the junction box which carries the starter solenoid and alternator field connections. If that is loose or corroded then that could cause a no-start / no-charge situation.
It does sound like a low voltage issue, and tbh I wouldn't trust the Yuasa as far as I can throw it. Their usual failure mode is to test just fine but be unable to supply decent current for starting. Does anything happen at all when you try to start it? Dimming dash lights etc? Its not uncommon for the F125 to prevent starting, but that wouldn't come with all the other symptoms.
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