If you're running on a stock A500, get "degrader" (I think it's on Aminet), load WB first, run degrader with "50hz" "50hzsystem" "survive reset". It will reboot. Remove the WB disk, insert game. PAL games should now display properly, although extreme top and bottom may be cut off - but this is better than a large chunk of the bottom cut off.
I don't know if this will work with the A520, but this is what I used to do on my NTSC A500 with 1084 monitor before I upgraded. After the upgrade, I now use WHDload which makes it easier.