I've had decent results with a 1/4" (6mm) end mill in a drill press (about $6-$7). You can make a "rotary table" for slotting sprockets with a piece of wood or metal dowel, the same size as the shoulder on the cam where the sprocket sits, mounting it into the center of a piece of 2x4 wood about 6" long. Clamp this to the drill table on the dowel, so you can spin the sprocket around it with the hole(s) right under the mill bit.
You'll want to slot it about 5 degress if using a K0-K6 cam, 8 degrees if using an "F" or K7-8 cam. Essentially, Honda dialed back the "K" cam 4 degrees to get the "F" engine, but it was done more to pass the EPA test than for power. They started with the dialed-back sprocket, at the time. When the F2 came out (or thereabouts) the sprocket changed and so did the cam, again, to fit it: the lobes were made steeper to open the valves faster, which ultimately ended up wearing the valve guides faster, too (part of the the F2-onward "valve problems").
Moving a "K" cam back about 4 degrees will add a little extra HP at 8500 RPM: how much HP depends on a lot of other things in the engine. Moving it forward about 3 degrees adds a real nice low-end surge that's useful in city traffic at the cost of a HP or maybe 2 at 8500 RPM. I've found through experimentation that many "F" riders prefer the cam moved forward, probably because that's where they "live" most of the time with the bike. Most riders don't spend a lot of time at 8500 RPM.
