The easiest way porting LK to classic is using data from LK alpha or beta version, they are mainly equal to TBC version. M2's are still in 0x0700 version, that could be easy converted to classic 0x0000 version with M2bc tool (posted somwhere on modcraft). Wmo models from alpha or beta clients are ready to use without any edits. Map ADT files are ready to use too, but without water. Water MCLQ chunk must be added manually (easy to implement in c).
The 3.3.5 m2's are more troublesome, because format changed much mainly animation system and particles, but are still possible to convert for classic one. I did simply tool for it (im newbie in c++ and have learned it while writing my tool), which converts only geometry, simply anims and collision. Im sure motivated person with average c++ knowledge could write conversion tool in one evening.
About 3.3.5 WMO's - hmm, siple models without inside parts - woks in vanilla without any edits. All more complex structures for example storwind or dalaran shows buggy in 1.12 client. Which means all outside walls are transparent. I think it is because MOCV chunk and outside flag part wmo files. It seems vanilla engine renders only mocv when indoor definition is set.
The map ADT files are usable in vanilla, MH2o chunk must be changed with MCLQ if you want have water on map.
ADT's saved with noggit 1.2 or 1.3 are ok for classic client.
Thats my quick overwiev. generally converting isn't hard , but requires more programs to use. if you are more intersted feel free to ask in more certain subject i will try to answer asmuch as my skills let's me
