An Aliens true form is pretty much the same as we saw in Alien.
As I've already stated in another topic, Aliens can't take too many aspects from their hosts, they only take few tarits, well one, their anatomical build, which means if their host has muscles, arms and legs, a head and bones, then Aleins will have muscles, arms legs a head and bones, but not much else, they wouldn't have a facial detail or take the muscle build of their single host, but rather the common muscle build of the host's race.
I say this because the Aliens could inpregnate sick, or immature hosts, that aren't full grown or have a sickly disease, and if the Aliens took enough traits, then they would be sickly, immature, or diseased.
Long story short, Aleins don't have original hosts, or true forms, they are meant to adapt, and all Aleins will share common bio mechanical appearences.