VCMI - it’s re-written from scratch, open source, multiplatform Heroes 3 engine with many improvements and mods manager. It requires some files from the original game though.
No variant rules unfortunately according to the Fdroid. You should check out the sudoku games from Cracking The Cryptic on the Google Play store if you want that. They aren’t FOSS but they have hundreds of handcrafted puzzles for a few bucks.