It seems that Pax Racing has a bad memory and has fudged the HP specs up to 12.2 with the FMF 4.1 pipe .....
In the link below they state ......[hey staggs racing we dyno the 150yx in a ssr bike and out of the box 10.4 hp, than put on a one off fmf 4.1 pipe and got 11.7 hp . this week end we are doing a 178 scut motor.]
GPX yx150 vs. OGM 150yx - Page 5 - Planet Minis
And ... we've been thru the hp thing before .....
What engine do you want?