If I was him, I would have tried to get a 650 motor, same weight, but with 30% more torque and HP that comes in a little lower in the rpm range.
At 6k rpm on a 500 you are probably getting about 40hp, on a 650 I bet you would be getting better than 55hp.
Also, he says it is a GL500 motor, so he is running a TI ignition and has a Volt meter.
I'd trust it.
EDIT: He should put some Harley stock mufflers on that thing.
the back pressure would do it good and the Sound would be cut by ~70%. it would add about 30# to the plane though.
Still, Soooooo much quieter.
He also needs longer intakes to make the carbs work correctly.
If you are building it from a kit, it should have a spec for the HP rating of the motors you want to run.Hmmm, I am looking for an engine that would work on my airplane project... Mine will be a little bigger than that airplane, maybe two GL engines? Or, maybe I should steal the engine from my Valkyrie?