im currently in the process of tearing down my 400ex. 
timing chain needs replaced, so going with a trued double roller (heavy duty, better than stock)     
never tore a atv down before, but isnt as bad as i expected.  jsut taking it slow, got a clymer manual to help out, and marking and setting my parts in order as i go.     
i was told there was alot of work done to this bike when i purchased it but didnt know.  now that i have gotten into it, i do know for sure that it is infact modified extensively.  but lack of a good timing chain.   
has a custom grind hotcams camshaft in it.  oversized valves and port and polished head.  pretty decent work done.   
there are some issues i will take care of though while im in here.  such as bearings and what not.   
ill post some pics up if anyone cares.     
i stopped for the night since i didnt have the right socket to pull the clutch basket off.