My understanding is that the EU has limited their ebikes to 250w motors and anything larger than than breaks the law. If you're not worried about that, get an upgraded motor and controller (and may as well do battery while you're at it). I'm relatively new to these, but purely from an economical and technical standpoint, these "fatty" bikes are not the most practical candidates for ebikes anyway. They're big, they're heavy, they have huge fat tires that create ridiculous amounts of rolling resistance compared to normal tires - all things that are not overly conducive to an electric bicycle. They work, it's just not the most efficient system. If you're dealing with a single speed bike, it's even more problematic because you can't gear up or down for more power.