Here is a circuit that allows you to connect any Atari type Joystick with 9Pin D connector to a 15pin PC game port:
As you can see, all you need is 3-4 small bjt transistors,a few resistors and the DB9 and DB15 male connectors. The adapter should work for any joystick/gamepad designed for Amiga, Atari, Atari ST, Commodore 64/128, Vic20, Amstrad CPC etc.
And here you can find a ready made adapter i also designed:
It's esseentially the same adapter but it's based on a Pic microcontroller instead.
Anyone interested, please contact at: firstname.lastname@example.org