Can TAS at different altitudes be calculated if only one performance point is given? If the POH only gives TAS at only one altitude, can we calculate the TAS at other altitudes? 

Here is a snippet from the POH of Remos GX. If we set the RPM to 5,200 then at 3,000 feet MSL ISA we get 107 knots. Can we calculate the TAS at 6,000 feet?
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.