I'm going to guess that the VS 1915 (at 85hp, according to their website) has a mild cam (Engle W100 or something similar), stock dual port heads, stock 1.1/1 rockers, dual 1 barrel carbs (34 Ict's?), revs to about 5,000 rpm with power and as Gordon said, is a good midrange performer. You could, as Robert suggested, put some small (36 or 40 mm) dual Dellortos or Webers with a better distributor on it, which together will make it noticeably smoother throughout the rpm range and nicer to drive (with a little more power in the top 500 or 1,000 rpm), but there's no kick in the pants horsepower increase here. A little, but nothing huge.
Some 1.25 rockers will add a bit more upper mid range/top end, and with the dual Webers/Del's (and new distributor if you choose) will be a modest but noticeable increase, not cost a fortune and be easy to do. You'll need new pushrods (different length) with the new rockers, and I recommend spending the extra $$$ and buy the high performance aluminum (Aircooled.net has some nice ones, as does apparently CB Performance). The engine will run quieter than with the chrome moly pushrods and, being lighter, the engine will rev that little bit higher (and be that little bit easier on the valve springs).
Some ported stock valve (35x32 mm) heads would go really well with the carbs and rockers, and together with a slight compression rise will now probably approach the power increase you're looking for. There are a number of people out there that can do a really decent ported head, and Steve Tims (Performance Workshop) does a "Super Stock" 37x32 head that's supposed to work really well (hearsay only; I've never actually seen or met someone running those heads). The temptation here will be to go to bigger valves as well- a lot of people will tell you it's a waste to put that much work into heads and not increase the valve size, but you're looking for driveability and throttle response throughout the whole rpm range here, not only upper end power at the expense of low end/mid range torque (where you spend 95% of your time driving).
All of this will cost in the neighborhood of 2,000- $2500 and add 15? 20? 25? horsepower, which you will feel and be a lot more fun. Alan is right, though- a 21-22-2300 cc, big valve'd stroker (140? 150? ???hp) has that much more power from idle on up and will make your car really rock!
think big, evil laugh here!
Hope this helps. Al