You can drive any 805x with an external clk source.Just connect the clock to xtal1 and leave xtal2 unconnected
A fundamental crystal is required for the onboard oscillator,but I think an external clock with 40/60 markspace ratio would work fine.

If you do ever need a clock with a 50% markspace ratio
then you can generate it by feeding a clock signal with any mark/space ratio into a divider,because they count falling or rising edges the output is always 50/50.