low power

As others have said, this is how physics work for all normally aspirated engines. (Turbocharged or supercharged engines do also lose power with altitude, at least eventually, but it's a far slower effect.) Your generator's engine also behaves the same way, which generally means the generator can't put out full rated power at high altitudes.

The solution, short of installing a different engine (or a turbocharger or supercharger), is basically just to put up with it. This may mean downshifting (or letting the transmission downshift if it's an automatic), running at higher throttle, and/or going at a slower speed.