This is a small patch to the fantastic Pellsson practice rom (https://github.com/pellsson/smb) allowing PAL version practice rather than NTSC.
//threecreepio
Go to the releases page to download the patch.
Then use an IPS tool (https://bbbradsmith.github.io/ipstool/, for example) and apply the patch.
Then simply apply that IPS (using for instance Lunar IPS) to the an original, unmodified version of the Super Mario Bros. ROM
The MD5 checksum for the ROM you should be using is 673913a23cd612daf5ad32d4085e0760