You can you bungee cords or rubber bands, although I have never done it because if you don't put it on right, it can cause a lot of stress to the charging rod, bending or breaking it. So I've heard atleast.
First of all, I agree. Second, you really don't need to change the spring; the spring is not "too weak" just use bands or mini bungees. I can get 100-130'+ with a good dart. That's with no spring replacement. And yes, it causes even mroe stress when just cocking the gun, and even more when firing.
But yes, if it cocks with the Crossbow spring, the spring is too long and cannot compress enough to have the catch along the plunger rod connect with the catch.