aluminium magnet wire

Not in a marine environment. And not when Aluminum is anodized, and corrosion isn't so much an issue considering there's a plastic coating over it in the first place. Also the primary reliability difference between aluminum and copper is contact points. If it's taken into design consideration properly (as it should be) it's just as reliable as copper. It's not used in home wiring anymore, but that's because someone cheap enough to want to pick aluminum over copper wiring is also going to be cheap on the labor and connectors and everything else. I think it's primary failing is solderability.

Nigel;
FOR HV overhead AC lines a special cable, called aluminum cable, steel reinforced (ACSR) is used.

The main reason to employ it is due to the large tensile strength provided by such arrangement, which is a must-have feature, to maximize the spacing between towers.

Aluminium is used in overhead conductors because, weight for weight, aluminium is about 40% more conductive than copper, and cheaper to boot.

The only downside as everyone has mentioned is that it's harder to make electrical connections to, although solders do exist that can do that and ultrasonic soldering irons can also make good connections as well as simpler crimping techniques.

Probably it was just used in this motor for cost; but it would also possibly reduce the angular momentum and definitely reduce the weight.

FWIW calcium metal is even better (!)