Punitive MDT already exists in CofCd20. Your MDT in that game is 10 ... no matter what level you are, if you take more than 10 hp in a single blow you might well die. Most 1st and 2nd level characters won't even have 10 hp to start with, so that only comes into play towards the mid-levels.
And if you run D20 Modern with the Call of Cthulhu d20 Sanity rules, you don't really even need to *convert* any of the NPCs in the BRP modules. D20 Modern and the D20 Menace Manual have many, many stock NPCs written up at 2nd, 4th, and 10th level. At most, all you'd have to do is substitute a few skills or feats to customize one of those generic NPCs to be whatever you needed it to be.
And, of course, D20 Modern is meant for a more cinematic style, but this can easily be addressed by using the rules for Ordinaries for PCs instead of just NPCs. Ordinaries have no talent trees, no bonus feats, and no action points.