We've used the term 'low-key' to describe Ad Hoc's David Gonzalez before. His latest release—an audacious Yamaha SR250—is anything but. This cheeky-looking scrambler was built to appeal to the urban and extreme sports sensibilities of its young owner. 'The first thing that came to mind was to make a motorcycle 'thug,'' David says of their initial meeting. So, from the Ad Hoc workshop in the peaceful hills just outside Barcelona, he set to work—starting by adapting the fuel tank from a Derbi…