Assuming you are not re-screwing into the same holes that were used previously, and assuming that the timber has not been damaged, or deteriorated from prolonged weather exposure, we don't see why the existing battens shouldn't be re-used if they were correctly sized in the first place. But perhaps Timber Queensland would have relevant experience, as we don't have any specific data on this issue. Their website will give you contact details http://www.timberqueensland.com.au/. 

The density of American red oak at 12% moisture content averages around 700 kg/m³. This means that in areas up to and including BAL-19 red oak can be used for window joinery, door frames and the framing surrounding any glazing that is less than 400 mm from an adjacent surface. In areas up to BAL-19, AS 3959 permits any timber with a density of 650 kg/m³ or greater in these locations.

It is surprising that reclaimed timber is not offered in a stress graded condition. Grading rules for recycled timber are available on our website and can be downloaded free of charge via this link https://www.woodsolutions.com.au/system/files/Recycled_Products_Structuralproducts_0.pdf. Span tables can also be downloaded once the grade is known. It will be necessary to know the species of timber before it can be graded, but presumably suppliers can provide this information.