Thought I'd post my own Cliffjumper mod.
Obviously the head had to be done, but I also wanted to give him the glass gas gun he's so famous for; it's on his bio, so why doesn't the figure have one?
A little while ago, I was searching for a weapon for my Classics Goldbug (elsewhere in this very subforum), and found that Universe Treadshot's pistol (which I had originally repainted for my Fusion Blackjack) suited him very nicely, and if positioned correctly, could also stash in his back window in vehicle mode. 'Course it's actually really hard to
get it in the right position for that, and even if you do, good luck getting it to
stay there while you transform him.
Last week, though, I noticed that the gun would actually slot nicely between his arms in vehicle mode, with the handle fitting in that space between the feet, and the peg that holds the knees together.
So I gave that gun to Goldbug, and decided to use the other one (Armada Sideswipe's) for Cliffjumper. But to make it different to GB's as well as to hint at the gas part, I figured it'd need a gas cylinder. A Space Marine melta-bomb, I thought, would be perfect. But could I find one? Could I heck as like!
So I wound up pinning a section from a Necron gauss flayer to the bottom instead. Still works...
CJ with Goldbug

Showing head mod and new gun

See that gap between the arms? That's where the gun goes.

And here it is, snugly nestled in place.