Forester and WRX are both built on the same platform so it's possible that they may be interchangeable... I won't confirm or deny that.

For the right price, anything is possible but it's not worth it. There was a gen2 for sale a few years back with a complete gen4 interior. It looked ok but it would've cost amazing amounts of money and because it was so hacked, re-sale value would be shot.

In 1990, there was the Liberty, Fiori, L series, Leone and Brumby. None of them would be an 'upgrade'.
Impreza/WRX's entered Aus in 1993 and Foresters in 1997. These will not suit your car. 

If you want to change the interior, pull some black plastic parts out, sand them back and paint them a different colour. Or, sell car and buy a newer one with an interior that you like