100 year old studs will be stronger than current studs by far. As long as they are not rotten or destroyed no need to do anything. And no wood hardener or oil of any sort should be used.

You shouldn't reinforce, however, since the wall is open, modern code-required firestopping between the studs would be inexpensive and reletively easy to install.

Ditto on stronger. However, often times the studs are probably not straight/bowed, which the original plaster probably hid somewhat. Going with drywall you may want to check that and sister as required.

The fireblocking would be if it is balloon framed, otherwise your top and bottom plates are the fireblock between floors.

We had a similar situation in our 1920 house. The studs are true (not nominal) 2x4s, and both straighter and denser than any framing lumber we can buy today. So strengthening should not be necessary.

Do look into upgrading your insulation - thermal insulation for exterior walls; sound-dampening for interior walls.

Your drywaller can likely shim or fur the studs if needed to make the walls more square/plumb. That was rarely necessary in our case, however (no more so than in the part of the house with new framing, new lumber, and experienced framers).