[S.D.A.]

I know this is quite stupid, but how do I restore the original spiny eggs' behavior in SMB? At what offset it is located?

[Jorpho]

See the links at https://tcrf.net/Super_Mario_Bros.#Unused_Spiny_Egg_Behavior .

[S.D.A.]

I need to do it on the EU version though.

[redmagejoe]

Apply the IPS to a US version of Super Mario Bros. Take the original US ROM and compare it in TinyHex to the newly patched US ROM. Find the only differences between the two, then copy the hex of the original and Ctrl+F it in the EU ROM. Chances are good that you'll find the exact block of hex, and then you just change it to the patched version of that block of hex.