I have seen a Youtube tutorial that advises one of the left-most diodes needs to be reversed for the circuit to work.
Having never yet built one of these, it still looks as though that advice is correct. At the moment, both diodes would perform exactly the same task, making one redundant. It looks like it should either be reversed or omitted.
You would need to replace the 6v motor with the 24v motor and use a 15amp power MOSFET on a heatsink to switch it. You would also need a voltage regulator to drop the voltage down for the 555, there's some simple diagrams on the Internet using LM317 regulator: