OEM replacement sending units should still be available, and nos/nors sending units pop up from time to time at the swaps. However, the issue here is that there isn't an electric sending unit that will work with a '55 head, as they all used the mechanical temp gauge. You'll either need to drill the temp sender hole out to the proper size or swap to the '56 or later head to make it work.
Just out of curiosity, is there a reason you want to change to the electric gauge?
