There are some D20 Modern critters that I'd like to port over to my homebrew, but I'm not sure how to approach this. Could someone please give me some advice?
Hm... The main thing I think you want to keep in mind is that d20 Modern is generally a much lower-magic game than D&D, so unless you're running a game with very little magic you might want to consider lowering the CRs. Defense becomes AC, but they're calculated the same way, so I don't think you need to change the number itself.