Cameron,
Before embarking on all that work and expenditure (getting another helmet) ask yourself what is your goal with your TK armor? If it is to become a member of the 501st and troop then you do not need to make all the modifications on your helmet. I was told by 501st Liaison Officer that the eFX helmet is accepted. If however you want to go EIB, screen original, etc..., then some changes are required (I believe).
I myself will change the lenses and add the screws on my eFX bucket. I may attempt to fill in the seam but will have to experiment with an acetone and abs mix.
Check out :
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for abs paste.
Cheers!