I would do one of the following
1. Get a 7 speed freewheel and source a cheap pair of shimano 7speed flat bar shifters like these http://cgi.ebay.com/Shimano-Easy-Fir...#ht_2368wt_905

2 find a used modern rear wheel, get a 9 speed casette and buy these
http://www.nashbar.com/bikes/Product...4_10000_202433