First thing u need to check is oil on plugs. if oil is not there then u need to check for leakages. If oil is reducing it has to go somewhere, lets evaluate possibilities, first ur rings r weak and it burns in cylinder so ur plugs will have oil residue, if the rings r now weak and no oil in cylinder then it must be leaking, lastly, no leakage no weak engine then it could be an overheating engine causing oil to burn, this could be,
. bad timing
. valve not seat
. bad oil
. somehow head has been faced and now internal dimensions r out
what did ur mechanic do with the head ? what oil r u using ? do u think the timing, mixture is ideally set and clutch plate is not worn