My UPSes absolutely hate power from gas generator during outages. Won't charge

I understand gas generators (especially older ones, like mine) put out rather choppy sine wave. There is probably frequency fluctuation too. My UPS completely discharged last time I had power outage for 24 hours. It just starts clicking, as if power is being turned on and off.

Is there a way to condition the power to the UPSes that comes out of the generator?

Generator I use is one of those portable Brigs & Straton ones that puts out 30A in 240V I have wired into the whole house via interlock.