Really neat figure, and I've always loved that design. The original toy was an absolute brick, so I decided to take a shot at a non-transforming ("Action Master") version.
The tiny Optimus is 'Legends' scale (the smallest size class above mini-cons/transforming weapons), and fits perfectly inside the dremeled out chest.
So far, I've incorporated parts from:
-TF Alternator 'Ford Truck' Optimus' head modified to fit a ball joint
-a KO Classics Nemesis Prime (Legs, back extension, part of the abs (that will house the balljoint, which came from a NECA figure)
-A recolored KO DIA City Commander armor (bulky robo-boots)
-TF: Prime FE Optimus (chest with back flaps opened/fused & window dremmeled out)
-Arms from a junker MP-01 Prime (shortened forearms) - you can see an earlier version of the torso in that pic -- a recast of the iGear Prime that I discovered wasn't the right proportion.
-Fakebuster Shapeway's 3D Printed hands (resculpted)
-Legends Prime with his legs severed at the knee. Dremeled out, and resculpted. Gaps in his legs are sculpted over. Superglued in a small revoltech joint for the knees.
The AM PM Prime figure fits in and holds great inside the chest. However, to accommodate the MP-01 arm joints inside, the legs need to parts-form off Legends prime and be set aside.