It appears to be a Getty's or at least the same style if a knock-off.
If the sellers was right about the 4500rpm, BTW, that is around the very top end rpm for a motor of that style.
Going by the 4500rpm, there is two ways you can discover the max voltage.
Either by back feeding the motor at a known rpm and measure the generated voltage and extrapolate the voltage at 4500rpm, or feed a known voltage into the motor and measure the resulting rpm and again extrapolate.
It appears to be around the 1.5~2hp mark, going by the photo.
I have posted some info before for detecting other specs, either search here or I can did it up again and repost.