That's a really cool experiment! The King engine has such a unique playing style, and it’s great that you found a way to run it in the Crafty GUI. Robert Hyatt’s integration of Polyglot into Crafty makes it pretty versatile, so it makes sense that it worked, but I imagine it might still be tricky to verify if The King is playing at full strength.
One way to check its performance might be to pit it against another engine at a known rating level in controlled time settings. If its results align with its expected strength, then it’s likely functioning properly. If not, it could be that some settings (like hash tables or book options) aren’t fully optimized.
It’s a shame that The King isn’t easily compatible with modern GUIs, but hearing about your success gives hope for retro-engine enthusiasts! Have you tried running any test matches against UCI engines yet?
I know a lot of people in the past have tried to run the Chessmaster engine in other programs with not much success so it was very difficult to run tournaments between The King engine and UCI engines. Just by curiosity, I ran the King engine in the Crafty GUI with no problems. Robert Hyatt built polyglot into the Crafty GUI so I guess that's the reason. Even though it seems to run fine I'm not sure if its playing at full strength. Anyway its the first GUI I've ever seen it run in.