I believe the original orange crush recipe used an orange oil emulsified with acacia gum. That, along with some citric acid should give you the taste you're looking for.
What? You don't have acacia gum lying around? Ok, boil some orange zest, and be careful not to get any white stuff. Also, for citric acid, you can try small amounts of lemon juice. If you have enough orange zest, it should mostly mask the lemon juice, but keep the tartness.