You would probably want to choose a middle of the road value for the
different properties and then let a CAE specialist figure out the specifics
for his or her simulations. Unless you make one part, a plastic piece will
not make much difference in terms of the CG if that is the purpose of the
material. In fact if you just use the density of aluminum as a base, it
will probably be OK.
There are cases where a guy will go buy something like a camera and give me
a 3d solid model of it. The only way to produce a good CG result is to go
weigh it and then create a dummy material for it with a density that works.