Skis are a great idea for one of those, anything to help it float better. Your clutching for 8000-9000 feet is caught between the charts, 5000-9000 and 9000-over.
For the non EXT at 5-9 is 63 gram cam arms (0746-694), 22tooth/13wide sprocket (0602-455), 40tooth sprocket (0702-393), 51' Helix (0148-222). 9-over is 57.5 gram cam arms (0746-702), everything else is the same.
For the EXT skip the gearing changes and use the cam arm and helix changes.
If your father-n-law is like mine you might go with the lighter cam arms and let it engage earlier for him, he probably won't over rev it anyway. Gearing it down is also something you might consider regardless of the EXT or not. I gear down all my stuff, and the 04' F6SP that my 15 year old son rides is geared down to help him get up and over stuff easier with out as much speed. Good luck...