I've had these sort of perverse thoughts about the Hiace camper van I may one day inherit from my parents (or at least inherit the responsibility for looking after it). Tarago would likely be a similar layout in the engine bay.
The current engine (3Y I think) is non-crossflow and so space on the drivers side of the engine is very tight. Meaning a big twincam head and a crossflow design would probably be very difficult to fit. Everything is packed in in front of the engine too, so extra length could well have hassles too.
Norbie's suggestion looks the goods. And for extra power sort some forced induction, easier since its fuel injected already.
If you do go ahead with it I'd be very interested in the results, so keep me posted.