You can use ISO Tool to decrypt your game and patch it. It's easy to use just select the game, select Patch, select yes Version Patch and presto its done.
But in all honesty if your PSP is a 1k/2k you should upgrade to 6.60 ME. It's easy to do as all you have to do is install 6.60 OFW and then run the 6.60 ME 1.8 OFW installer and in less than 2 seconds you're patched.