there is no given answer to how much you can remove. the best way to shave the head is to take a thin piece of lead (such as soder, make sure it is flux free) and shape it into an "L" shape. remove the spark plug an place the lead down into the cyil. making shure to reach the quench band. Then slowly turn the motor over. the compressed thickness of the soder will give you a base measure.
depending on the build i wouldnt recommend going down past .060" quench band (unless you know what your doing and dont mind risking your motor).
take your head off the bike, pull the allignment pin out CAREFULLY. lay some fine sanding paper on a planning serface (could be a thick glass, at least 3/4" but i use a good planning serface made from granite.)
KEEP IN MIND, you can always remove more later. dont get too greedy. i hope this helps, PLEASE only use this a a referance if you dont feel comfortable doing somthing dont do it.
have fun turning you 2 stroke into a 4