The Prius and Corolla engine may be similar, but they are far from the same engine. The older Prius engine was based off the Echo/Yaris 1.5L, but again it was quite a bit different. The hybrid system adds a ton of complexity to the drivetrain.
DIY hybrids are not an easy undertaking (thus the lack of them actually being done). I believe I have only seen one conversion and that is coyotex's Geo Metro conversion that is documented here. I'd check that out as a place to start getting ideas.
Geo Metro plug-in parallel hybrid EV - in progress