They're about if you really want to chase them. They're not a true diesel, I think they're actually a midget hot bulb engine. Interestingly, UK moped regs state 50cc i/c engine for 2 wheelers, but trikes/quads satisfying moped tax/speed regs have a 4kw i/c or 50 petrol restriction. So you could run a 5hp diesel in a trike if you wanted to, thus giving you, in contrast to the Indonesian petrol tuk tuk, the knock knock.