Wouldn't be too difficult. Poincaré/Lorentz/Hilbert were close to developing the same concepts behind special relativity around 1905 as well. If Einstein randomly died in 1904, I think relativity would have been discovered within the decade anyways, just by combining their knowledge.

Lorentz developed the Lorentz contraction independently of Einstein already, he was just hampered by the fact he adhered to the idea of the luminiferous ether as a medium for light propagation. I fully believe Hilbert+knowledge of tensors (Einstein didn't know the concept of tensors in 1905! [1]) would have developed general relativity as well.

[1] Einstein actually had an idea for general relativity much before he actually figured it out, he literally just lacked the mathematical knowledge to formalize it. He had to learn tensors in order to develop the Einstein Field Equations https://www.quora.com/How-did-Einstein-get-the-idea-that-he-...