Comparison of copper and aluminum winding wire

1.A calculation for aluminium and copper wires.
Copper has lower resistance per unit volume, ie, a 1 cm diameter copper wire has less resistance than a 1 cm aluminum wire.

BUT, aluminum is much lighter than Cu for the same diameter. If you used an aluminum wire that weighs the same as the 1 cm diameter copper wire, it would have LOWER resistance. And that is

why aluminum is used in power transmission, and also in aircraft, where weight is important.
To do this you calculate density resistivity which is (resistivity x density). Here is a partial table:
Al 28.2e-9 Ω-m * 2700 kg/m³ = 76e-6 Ω-kg/m²
Cu 17.2e-9 Ω-m * 8960 kg/m³ = 154e-6 Ω-kg/m²
Ag 15.9e-9 Ω-m * 10500 kg/m³ = 167e-6 Ω-kg/m²
Au 22.14e-9 Ω-m * 19300 kg/m³ = 427e-6 Ω-kg/m²
Zn 68e-9 Ω-m * 7130 kg/m³ = 485-6 Ω-kg/m²
Fe 100e-9 Ω-m * 7870 kg/m³ = 787e-6 Ω-kg/m²

Note that Al is half that of Cu.

2.How to replace the aluminum winding wire to copper wire in one transformer?
Since aluminum wire is slightly more than copper you'll need to account for the change in voltage drop.
If you replace just one half of the transformer with copper wire you'll need a resistor in series (series resistors add to the resistance).
If you replace both sides with copper wiring then you will not need a resistor to account for the changes.
Or you can purchase a new transformer. And you should expect that with an equivalent rating,
a transformer with copper winding will cost more than a transformer with aluminum winding.