I have to ask, and not to be rude, but anyone who has the money for such a new RR, even a used one, normally would take their car to a certified garage for such menial tasks ?
With that said, I went to RockAuto just to see, and there is one listed, but no photo. It's a NISSENS 60779, never heard of that brand though.
Googling that shows no photo either, but a rad is rad, there has to be a spigot somewhere to drain it.
There should also be a drain on the block and for a W12 or V8, there usually is one on each side of the block.
I would think the real problem of working on such an engine are all the plastic shrouds covering everything up, it may take half a day just to get at the drain cocks.