Wouldn't it be better to sell them the plans then? Reverse engineering sounds like a bitch.
I'm still not fully engrossed in the story thus far due to the way Maou seems to be some sort of genius at everything except fighting and love. Inventing, farming, economics.. I'm with Hero here.Why does she even need him around to save the world?* see edit. As (Ryll?) mentioned before, we haven't covered the problems with the Demon world yet. If we're to assume that the farming and crops originated from there (to make Maou's execution a bit more grounded in reality), exactly what problems are being alleviated by having a war for them?
edit: Okay, just remembered he's going around kicking butt. So the humans need some brains to solve their problems while the Demon world needs some muscle to keep things in order? Well.. that makes things easy :s. If that's the case, Maou and Hero should just swap places.
edit2: Maybe I'm just taking things the wrong way due to my prior perception that Maou is here to solve economic/people problems, not technological ones.