problems with mesh deformity when importing from blender

Hello, i have created an enemy model for a project, however when i import it to unity it has a partially transparent head that always shows the back of the head. by this i mean it shows the backside of the head whatever angle you are looking from. look from above, and you see the lower half of his head, look from the front and you see the regular back of his head.

it also deforms, and displaces a part of his arm.

i have recalculated the normals in blender as wel as inverting some normals where i think needed which has solved other problems (before recalculating it was transparent at alot of places) however these 2 i cannot get rid of.

thanks in advance