If you want Magnemite to evolve with the use of an item, you need to make up a new item.
If it could evolve with a Thunderstone, it would've happened back in Gen I, as a Thunderstone existed then.
Therefore Magnemite cannot evolve with the use of a Thunderstone.
Another thing, I think we should leave the Gen I alone from now on.
Too many pre-evos/evos. have been made for Gen I.
I'd like it to stay as original as it can be.
Just a thought.