You'll probably need more than an M120 for 450fps, I'd have thought it'd need at least an M130 or 140.
It's a bit of a risk making such a huge fps jump just by upgrading the spring alone, some of the parts might not be built to take much more strain than the 320fps spring that the gun comes with.
If you don't want to upgrade anything else, I'd suggest upgrading to an M120 as you said, but also getting a 6.01mm tight bore barrel. The M120 springs are rated to 390fps, then a tight bore will give an fps boost of around 20 - 40fps depending on the length.
Doing it this way means the internals are under less strain than just whacking a massive spring in the gun to get the desired fps, the longevity of the parts will be better doing things this way, you'll probably see an accuracy enhancement from the barrel as well.
Doing that ought to get you at least into the 400s. Picking and choosing your fps is far from easy though, there are a lot of things to take into account.
