There was an article in ETI Electronics, Vol 27 issue 5 (24 April - 21 May 1998). It discusses Mag readers, and also has a Pascal program for reading the cards in a PC. You could convert the program. The project plugged into the Game port of the PC. I have the article here, if you want me to scan or something (only 3 pages long). It worked really well on the PC, but never got to rewrite for the 8031. It used an Omron 3S4YR-HNR-4U reader, but probably all the same.

Pinouts were as follows:
1) n.c
2) n.c
3) n.c
4) RDP Read Data Pulse
5) RCP Read Clock Pulse
6) CLS Card Loading Signal
7) CSV Current Save
8) +5v
9) GND

Cheers