Unfortunately Flash is inherently insecure which is why it is being “decommissioned” and withdrawn.
Guessing that you can’t persuade the author to re-write the game in Javascript/CSS/HTML5 (if the game is 18 years old just finding the author will be a challenge) I assume that by "tried alternatives you mean the ones listed at Open source alternatives to Flash Player | Opensource.com
If those don’t work you might be out of luck.