Reshaping a helmet?

[Manius Acilius Italicus]

I build helmets and would be redicent to try "squishing" it. Some of the ideas, like Robert's could be good, however a lot would depend on the material and its thickness, quality of construction and the attachments on it. I tried adjusting a helmet I was making by distorting it and it altered the neck guard and the brow guard no longer fit the curvature. In your case if you could remove the brow guard it may become too short to align with the holes.
My honest advise would be to find someone it fits, sell it and buy another fitted to your head. Just my opinion, but please don't lose the money you have invested already by messing it up. You may get lucky, but then again, you may not.