Honestly, with the Phantom engine, I would leave well enough alone. You would be surprised just how powerful that little engine is out of the box. It's everything everybody wishes the normal 66 and 80cc 2 strokes are in terms of power. 45mph is achievable by pretty much everybody right out of the box with the super simple and easy to tune Bofeng carb. More importantly, it peaks it's torque very low in the RPM range, so it climbs hills and takes off from a stop better than pretty much everything else on the market.
Basically, install it and run it as is first. Get through the break-in period. Then, if you decide it's still not enough for you look into doing upgrades.
Basically, install it and run it as is first. Get through the break-in period. Then, if you decide it's still not enough for you look into doing upgrades.