Would you be able to say morph Willbender when Warp World resolves? I don't believe you can because you aren't casting the card, instead you are simply putting it into play.
Any cards put onto the battlefield with Warp World enter the battlefield face up, since Warp World doesn't specify otherwise (C.R. 110.5b). Thus, since Willbender will enter face up, you can't turn it face up by paying its morph cost (under C.R. 702.36e). Neither will Willbender's triggered ability trigger when it enters the battlefield (C.R. 603.2).
EDIT (Jul. 9, 2019): One rule was renumbered with Core Set 2020.
EDIT (Jul. 9, 2019): One rule was renumbered with Core Set 2020.