So would you think that a system with a 3 brush generator would be intelligent enough?
I am pretty sure a 3-brush system, combined with the vibration and heat of a vintage motorbike, would completely destroy a lithium-ion battery, if it is anything like my 1942 WLA Harley. (Pretty much same system as Indian.)
I have never used lithium-ion but plenty in my circle of biking friends have and failures are legion, even on modern bikes. They do not seem to be able to handle vibration and heat well at all.
And the 3-brush system as used by Harley et al has no regulator at all. Only a cutout switch that does not regulate either volts or amps. The generator relies on positioning the 3rd brush to manually to set the maximum output and hope for the best. Originally this system relied on excess current boiling away the electrolyte in the large wet-cell battery to get rid of the excess energy. Manual says to top the battery up every week for this reason.
As the original large wet cell batteries are no longer available where I live, and apparently the latest versions available in the USA are poorly made in China and fail frequently, I have gone to using sealed AGM batteries often used in commercial building fire alarms and UPS’s etc. But even these have given trouble swelling up and failing due to over charging even with the 3rd brush in the lowest output setting. A check found voltage spikes of up to 17 volts coming out of the 6-volt generator!
In the end I have gone to using a solid state V-tronic voltage regulator from Samwell Supplies in the USA. https://www.samsup.com/product/regulator-electronic6v-or-relay-style-no-cover-for-2-brush-generator-only/
With this regulator, the output is a dead steady 7.2 volts and so far the small AGM batteries have survived without the incessant failures of the past few years. You do have to change the internal wiring of the generator to a 2-brush configuration though. The regulator fits inside the original cutout switch housing so is not visible at all. Looks all original.
So, if you have found an ordinary non-lithium ion battery that actually works and survived since 2015 with your 3-brush system, I would stick with that.