I doubt that Eventers will really offer much of a differing opinion ...
However should you be willing to shave your head (donate your hair to cancer patients!) I suspect the helmet will fit quite nicely IF it's the correct shape for your head ... by the time your hair grows back in, the foam shell will likely be sufficiently compressed that it will fit quite fine
(liner compression is typical for helmets if they are worn frequently enough)
If you're able to try the particular helmet, wear it around the house/shop for a while - the intensity of your headache will clarify the decision
Fortunately a pixie cut is likely all you need do (unless your hair is already very short)