I am interested in this too. What are your feelings, Noel?
My own particular prejudice is that modern helmets look anachronistic and out of scale in a cyclekart, whereas old fashioned pudding basin helmets look right. I know that at track days some drivers do not wear helmets at all, so that if older helmets don't provide as much protection as modern helmets, at least they are better than nothing, while being in keeping.
Fat Skeleton sell helmets which are sized, so that they are much more proportional. Other helmets tend to be universal sizes with various sizes of inserts to make them fit smaller heads.
Ironically, a potentially very tired 1960s helmet with a kite mark is legal (kind of grandfathers' rights, I suppose), while a newly made Davida pudding basin helmet is not legal because it doesn't meet current specs. I wore a '60s one to the Isle of Wight scooter rally in 1999 on a Lambretta and got no grief.
For cyclekarting, if it wouldn't be a problem with the club, I shall probably use a pudding basin - or even, if some are wearing nothing, a leather flying helmet.