Can TAS at different altitudes be calculated if only one performance point is given?
If you keep 5,200 rpm at 6,000’, the engine will be producing slightly less power due to density altitude. There will also be slightly less drag so the speed will only drop about 1-2 knots. Because the engine is producing less power, fuel consumption will be slightly less and range will also increase.

The performance figures for a Cessna 150 can give you an approximation of how the numbers change with altitude.