Lithium batteries can't be normally be used as a direct replacement for a lead-acid battery, they require different chargers.
They often survive long enough though, as in this case, but... under some circumstances they might get overcharged and protest in the form of an explosion... and when lithium batteries explode they are quite little napalm charges...
To sum it up, they can work in the place of lead-acids but they're not safe, more precisely, they can be quite dangerous!
To correctly use them in a vehicle originally equipped with lead-acid batteries, proper charge electronics should be fitted.