My first computer was an Atari 400, great days. I never played decathlon though, so I didn’t start customising joysticks until later, I think when I had an Amiga. I got a proper arcade joystick and buttons and built them into a box like the x-arcade sticks you can get now, and wired the contacts onto an old keyboard so the computer thought keys were being pressed. Lots of trial and error figuring out which combinations of keys the keyboard controller couldn’t detect together, but it worked really well and got years of use.