Yeah, seems Diesel is keeping his counsel to himself- or jealously guarding his Copyright.
Sorry, not a very original stunt, obviously. As already noted, just siphon it out into a jerry can and fill the tank with petrol. It will be a bit smoky, but you could fill it with petrol again when it's half empty to dilute it even further.
The ST13 has a lower tank holding 5 litres. This isn't readily accessible without taking the top off it (with the seat removed for access). Thinking about it, you'll need to go there and empty it because that's the source for the engine, and it won't mix with the main tank. You could have a look at the on-line Manual before you start.