Here is a detailed guide to help anyone who wants to rebuild PSP games. With VCDROMX rebuilded games always works without errors.
You can use UMD Gen but some games doesn't work.With VCDROMX English (or Chinese if you prefer, download both here) I have never had that happen.
Open the file and select where winrar extract the iso image and hit extract.
Using the VCDROMX Extension tool, drag and drop the extracted files and folders.
From menu go to File -> Build VCDrom to make the FCD file
For volume label put the name of the game, as VCDROM type choose the first and as filename put everything you want. Press OK to start.
When finish, open up the VCDROMX Convertor tool, select *.FCD type under the "Source" pane, and select the FCD file you just made.In the lower window, select ISO type and select the Rename and Overwrite box under "Target" and you're all set!
Now choose convert to start.