Scooterworks USA Big Valve Head (2 valve, 61mm); GY6
Not legal for use in California on pollution controlled vehicles. Check local laws before installing.
Almost the exact same one I used. Excellent alternative and the same performance.
See the build series
https://www.youtube.com/playlist?list=PLKwd-ZcQF16zZuQ4kzvtUdLR4i7HNRTGT