If you don't have a save from before the wastelands, 'cheating' is the only way you will be able to get the stuff you left behind. The one exception is the teleporter stones; they get replaced anyway if you bring them with you, and if left behind you can find a set of ancient pyramids near the beginning of the wastelands (screenshot
If you do not want to try getting back to Rivellon, there is an exploit which will at least let you replace the gold or potions left behind (works with anything stackable).Spoiler : Drag a stack out of (or into) your inventory (or container) and hold down the control key before dropping. This will bring up the split stack pane; type in a negative number and hit enter (the checkmark will be greyed out). This can also be done in the trade window.
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Don't think of it as cheating, but more like retroactively correcting a game design flaw, where you were not adequately warned about the one way nature of the trip to the wastelands.