Weight wise the raised guideway was built to mainline standards so it could support the weight, even of a freight train.
The current station could not support a 12 car bilevel as its not long enough.
However, considering that GO RER is in motion, it would be seriously unwise to see a 12 bilevel car stopping at the airport.
The bilevels will not be stopping at any stations within the inner GTA, period, let alone the airport, once RER is implemented. So why have it stop at the airport?
Round holes need round pegs. Stop trying to force a square peg into it!