I changed my eye bolts this year (275 no.59)and was surprised by by the amount of corrosion. I obtained replacements by return of post from Parkers for a few pounds. The old Eyebolts turned by pushing a screwdriver through what was left of the metal and fitting a long extension pipe over the handle. The bolt thread was as good as new with the grease still on the shining threads.
The latter was advised in an old forum article and works well.
The reason the eye bolts are cast iron I suspect is due the strength required under load. Also other metals might cause a re-action if dissimilar metals are used.
I wish all the luck that I had
Mpollard