Even at your altitudes, I think you will find the performance of the 100 hp 912 to be just fine in what should be a very light weight plane,
That might depend on the mission and the density altitude at the time. We very often have 8000'+ density altitude which would give that engine max at takeoff 76hp (for 5 minutes). Drop that to max sustained at 5500rpm and it's closer to 70. The 912hp curve drops off even quicker from there. Cruise at 4000 and it's down to 67hp (s.l.) and closer to 51hp at 8000 d.a.

That might work for some.