A new derailleur would fix it, but friction bar end shifters are soooo much easier, and available in higher quality versions than the Sora shifters currently installed... and would be more 'period correct' for the bicycle (not that it matters).

Here is a set of very nice shifters:
http://www.rivbike.com/products/show...end-kit/17-089

You couls also get a set of 7 - speed indexed SHimano shifters used off EPay, but to use the indexing you would need to insure compatibility of derailleurs, cassettes, chains, and cables, just like with the Sora ones.