With a Gameport to USB adapter, you still having the limitations of the old joystick, the potentiometers wired as resistance, more "noise", few buttons, probable 4 bits of resolution...
Wiring the old joystick "mechanics" in a new USB controller, you improve the resolution to 12 bits, can replace the pot' with HALL sensor, can use more buttons, axis, filters...and can end with a better joystick than these low cost that you can buy.
Anyway the best Gameport>USB adapter is Rockfire, for the price dont worth building your own:
