Its interesting to see the catch designs of this compared to those of historic crossbows. Granted, we're not working with a draw weight heavy enough to warrant a lever system, but its still cool none the less.
If you're going to want to stretch the spring (ie if you have no other springs on hand) you should definitely "heat treat it" using a propane torch or some other tool. This keeps the spring at the stretched length and keeps it from deforming. True, if you stretch the spring (without treating it), performance will increase by a good amount, but as you use the blaster, its performance will decrease quite a bit as the spring deforms.
I wouldn't recommend trying to heat treat a spring on your own unless you've had training in the process and a whole lot of practice. Heat treating and tempering is a pretty complex process that even professional blacksmiths screw up from time to time, and without computer aided forges/kilns and quench tanks it all comes down to rules of thumb. Do yourself a favor and just buy a new spring.