A great idea when turning them into zombies is to think outside the box.
Look at skaven and the old undead art work.
there is rotton flesh and exposed bone everywhere but more importantly, there is tubling, cables, leg splints, cobbled together weapons and god knows what everywhere.
Whats that?... You say your ogre lost a hand?... Replace it with a trident running up through his forearm so the head can stick from the exposed stump, tridents not your thing?... that cool use a sledge hammer head.
Your ogre lost a leg?... Peg leg or better yet give him wheels.
Also another favourite, have hisexposed stomach being used as a home to some little gribly like a giant rat, bat or whatever.
The joy with working on a modle as big as a ogre is that not many parts are out of the question, nearly everything works for converting, is just kit bashing you can't do so much.