Based on the retconed history of The Spirits of Vengence (Ghost Rider #33), this is my rendition of the "Undead G-man"*.
-MM Joker Goon
-ML Professor X (Jacket)
-Megablocks Skeleton (Head/Neck)
-Misc. Flames, recut/assembled
-Mezco The Spirit (Tie)
For basic reference below, I've included the closest source image I could locate from that run - a shot of 'Undead Gman'.
* Before posting this, I could *swear* I had a saved scan from one of the Ghost Rider books of this exactly character (skull belt-buckle, black suit and all, done in a very digital 'painted' style - possibly an interior full page) - however, after looking for that .jpg - it's nowhere to be found. That's what happens when you make something, and don't photograph it for 2 years. Your reference art disappears. ;) If someone can find a shot of this actual ghost rider (or can tell me what issue he appears in) please let me know.