First and foremost....Square off the intake a exhaust ports, taking a 90° sided file and adding more area to the space that the gas will enter and leave the cylinder. Second, I think the 630 is a simple piston port design, meaning if you want even more gas to enter the cylinder, you can file a square chute, the width of the intake port about 40 thousands of an inch into the top of the piston. This will allow the gas to enter in sooner and it will suck more into the combustion chamber. Then, look with a micro mirror, into the intake and exhaust passages to make sure all gaskets are trimmed even with the metal castings. At the same time, look for metal casting lines that can cause fuel and gas to hang up. Take a Dremel and kncok them down to the other side of the port. After that is done, polish the port so it shines. This will allow gas to enter and leave faster.
Then we have the illustrious muffler system. Take it off and install an small expansion chamber, it will kind of look like a two reverse cones welded together, and use a small silencer on the discharge pipe.
This should give you an additional 1500 RPM's and am HP gain of 25%, however, tweeking the ports wil make the engine run on a power band, and, it will bog until you hit that band. We have a 630 in a 4' long 1.5' long RC model of Miss Budweiser and these modificatons allowed the boat to run from the 50MPH it did to 62MPH it does now.