Can Stockfish (with small modifications) be used for other games now? There are a few decent open source AlphaZero implementations and I wonder how it would compare.
Depends on what games and what you define as “small” modifications. There are a lot of game-specific tweaks in the search code.
You can still apply alpha-beta tree search to lots of games though.
NNUE is the interesting part to me. Alpha-beta tree search is useless without a good value function. Not sure what would be the best way to generate the training data if you're starting from scratch.
One way is through self-play. You can pit two value functions against each other to determine which is better, thereby learning the value function.