Are you going to fill us in on how you did it for anyone down the line who also asks this question or is it a secret?
I'll be posting how I did it later in my Customs thread.
name's roxaclock, see ya around!
Good on you, FfarDev. Turns out my guess was correct and relevant to all releases of him (thanks again, Chris). Then again, I'm not too surprised given most faces/different coloured parts are moulded separately. Now we'll just have to see how you'll shrink the mould for the new cast after filling the space.
P.S., I noticed Rheneas now has regular coupling rods, but his fake valve gear is still there. Looks better than other versions I've seen of this mod where the whole thing is removed and they don't reach the cylinders.