"Problem is that the motorola is putting out close to 15volts"
"Any suggestions on what to do here? "
Yes. First, disconnect that alternator to avoid damaging those batteries. Then find out what voltage they need, from the mfr. And take that alternator out to have the internal regulator replaced, if the gels can take a standard 14.3-14.4 volts. If they need something special, you can easily modify the alternator (or have it modified) to use an external regulator which will charge them properly.
If the budget
gods are generous, I would suggest modifying the Motorola to use an external alternator, and then buying
a second Balmar 614, or something that is pin-compatible with it. That way IF your primary regulator goes south, you can steal the one from the starter and use it as a hot spare.
It is possible that someone had the Motorola internally modified in the past, to accomodate the voltage loss from the isolation diode, and you may just need to "unmodify" whatever was done. (i.e. there may be an extra diode inside it, to be removed.)
Better to fix it right than to kludge around. And to label & document the system afterwards, because years later, we tend to forget about this stuff.