If it were me at a mile high, I'd go with a YS110, or a 120AX or Saito 125. If you don't already have an engine, that's where I'd start.
The 120AX is only an ounce more than the 95 and if you want stupid power, there it is.

Just keep in mind that weight is just as important at high altitude as having enough power.
The airframe acts like is has less wing area along with being heavier.