I have found that my DS II sails pretty good even in light air with just the standard main and jib. My sails are pretty "blown-out" (heck after 31 years.... I guess that they would be!) but I still don't do that bad.
I would say if you really want to add a light air headsail, that maybe look for a drifter or asymetrical spinnaker from a similar sized boat, there are no light-air sails allowed in the DS Class races (just main, jib, and spinnaker) so no one (that I know of?) makes a specific genoa or drifter for the DS. Perhaps a Nordica 16 genoa would work, it will require adding fairleads for the sheets since the standard DS jib-sheet fairleads/cleats would be too far forward for a larger sail (unless it was cut to use those fittings, not sure if that would allow a much bigger sail than the standard working jib).
The earlier DS models (prior to 1974) did use an end-boom mainsheet, so you could set your newer boat up like that or install a traveller in hte cockpit to allow moving the lower mainsheet block to windward for closer sheeting in. I'd suggest adding a boom vang if you don't already have one, it holds the boom down making it easier to sheet the sail in for beating and also prevents the boom from lifting while sailing off the wind giving better control of the sailset. One other idea that I intend to add eventually is to upgrade the mainsheet to a 4-part, it will require replacing both blocks that came with the boat in order to add the 4th part. That should give more power to the mainsheet and reduce the effort of trimming the sail.