I made an HTML pen-and-paper demo here. You can store facts on paper and use Femto-FSRS for scheduling, just like Piotr Wozniak did in the 80s.
This is supposed to be a minimalistic library that can be used as a starting point for FSRS-enhanced apps. If you need a more full-featured offering with features like logs (so you can optimize your w param) or revert, etc.., check out ts-fsrs.