Standalone AGS Module?

I have a potential customer with some off brand inverters and lithium batteries.

The inverters do not appear to have an AGS routine, or connection of any kind.

They do have generator input termnials.

Is there a recommendation for an AGS that monitors battery voltage (or better yet SOC by the data bus but I doubt this exists yet) and will cause a generator start/stop?

I figure it might be good enough to trigger a start when voltage starts to drop off, and run for x amount of hours and then stop.