the big issue is csgo runs on source 1 and cs2 is source 2, completely different engines so the model format isnt the same. you cant just drop the old model in, youll need to decompile it, convert the mesh to something source 2 can read like fbx or smd, then re-rig it to a cs2 skeleton and recompile with the source 2 asset compile, you can do it, it will just take some work. you can decompile the csgo model with crowbar or similar but i have never done that for cs2 only for csgo to csgo with different versions.