More details. Is this a ‘fun’ charity-type event or a Fondo/Sportive? What are the longest and steepest grades? You could put a smaller gear on the single speed if most of the time is being spent climbing, and you can coast downhill on single speed. You’ll give up some speed on the flats but if there is truly an large amount of climbing this won’t matter too much in the overall scheme of things. With less than two weeks to go you’re almost certainly better off on a bike you know rather than trying to set up a new bike.