For those that want to nerd out, here's how I've come up with that 5hp number.
First, I applied a simple Kalman filter to each axis of accelerometer data - I suppose it's a 1-D filter, with an adjustable parameter for sensitivity. There's still some noise in the data, but smoothing it out much more led to inaccuracy. Here's what the raw data for all three axes looks like, for reference:
After filtering the raw accel data, I subtracted out Z-axis (vertical) and Y-axis (longitudinal) steady-state readings. I got these by averaging about 20 seconds of the top-speed portion of the run. The camera was mostly upright, but tilted forward enough that the Y readings were about 4m/s/s steady-state. Also, this means that my acceleration (from going WOT) was registering on both Z and Y axes. I then combined the Z and Y axis data, where total = sqrt(z^2 + y^2). This gave a pretty reasonable approximation of the acceleration.
From there, I did a wild guess on total weight, I said 127kg. I'm 180lb, I assume the bike is a bit more than 100lb. Anyway, close enough for mopeds. Since F=ma, I can then calculate the force for the acceleration. Since Power = Force * velocity, and I have the GPS speed data, I can calculate the power required to give that acceleration. Note that this is actual power to the wheel - even though the engine is at 10,000rpm at launch and probably making ~4hp, NONE of the power gets to the wheel when you're not moving (since velocity is zero!). It all is going to heating the clutch. The torque, however, is easily calculated from the acceleration-derived force, using tire radius, sprocket sizes, and primary reduction (total gear ratio is about 21:1 right now).
I'm not going super fast, but there is a bit of aerodynamic drag involved, too. Using some rough empirical numbers, I used 0.675m^2 for frontal area, and 0.65 for Cd, giving a 0.44CdA value. Used the conditions of the day to estimate about 0.7kg/m^3 air density, and Drag = 1/2 * density * velocity^2 * CdA. Multiply by velocity again to get power. Add drag power to the acceleration power to get total power to the wheel.
All calculations are summarized in this plot: