Decrypting the sudoku ticket file isn't
too complicated, there's a bunch of AES decryption source code around (I think twltool, nus downloader, srl extract are all open source, and gbatek contains AES pseudo code). Once when you have the decrypted ticket, then you can simply download the old sudoko version via nus downloader.
@WulfyStylez: Would that be a feature for next twltool version, decrypting .tik files with per-console keys?
Once when you have the old sudoku version, and downgraded the firmware, then you will only need the .sav file. Theoretically, the 'traditional' way should still work:
http://hackmii.com/2011/01/sudokuhax-release/ (ie. export the game to SD card, use that injection tool, and then import the game back from SD card to DSi) (and when done, you may want to copy the .sav file from DSi to PC, so you won't need the injection tool in future). I would be interested if that's working, too, since it would be the only way to get the european sudokuhax v1.1 .sav file (which was announced to be booting faster than v1.0)