I just got an e-mail back from Billy Goat. I was really impressed how quickly they responded to my request and how helpful they were. They said :
"There are two good options that we are using on our current production units. The first is the Honda 13 HP [ GX390K1QAA2 ] and the other is a 14 HP Subaru [ EX400DE5030 ]."
Looks like I will be shopping around for the Honda. Haven't heard much about the Subaru motors. I have the 9hp Honda on my Billy Goat push blower and love it.