Suitable Shimano integrated shifter/brake replacement for old STX
The ST EF-51's have a little switch in the back of them to switch between V-brake and cantilever. I found them too low quality to use reliably though. 

You have several options:


Go to 8 speed or more and pick up some Alivio shifters. This is what I did on a 7 speed bike, and this is what I'd recommend. 
Microshift makes some 7 speed shifters that are shimano compatible. Buy your brake levers separately. If you're not going to go 8+ speed and want indexed shifting, I'd probably try this (even though I haven't seen these used, the ST EF-51's lasted so short for me that I'd be willing to try this instead).
Friction shifters always work. Add your favorite cantilever brake levers.
SRAM Attack! 7 speed (twist) shifters work with Shimano and aren't too hard to find. Add your own brake levers. 
Find a better grade Shimano 7 speed shifter (somebody might have old stock in a decent bike shop). Some may have V-brake only (see 6) or cantilever or bring your own brake lever. 
V-brake levers can work with cantilever brakes with the aide of a travel agent (a pulley to change the cable pull). Alternatively, you can replace the corresponding cantilever brake with a V-brake. I'd buy a V-brake rather than the travel agent (it's cheaper and easier, e.g. Avid Single Digit 5 or 7).