Something has gotten between insulation and metal of oven. Whatever it was, it was cremated and the smell is bad when oven is turned on. Can the insulation blanket be replaced?
I've done this (due to rat pee (which is at least as bad as it sounds)). It is a straightforward DIY job, just be sure you have good screw management solution. I prefer to tape screws near the hole they came from.
Just get thin unfaced fiberglass batts in the right size. Remember that many call fiberglass the new asbestos, and wear a minimum P100 mask and non-fabric gloves.
You can also try running an "oven cleaning" cycle and cook the corpse to carbon black. If it's just a corpse you have a good chance the smell will dissipate, at least if the corpse is really right against the oven wall.