Get original xperia play content back on device after removing it
You can use their official update software, it has been called "PC Companion" the last time I used it:


Boot your phone to flashmode (See also  How do I turn on flashmode in Sony Ericsson Ray?)
Update/restore it with PC Companion  


This is mostly for modding, so it won't be that useful to you:
You can also flash a .ftf file of your choice (official or a custom mod) with FlashTool, note however that you need to unlock the bootloader before your phone will boot a mod with an unoriginal kernel. I have however no idea where to find an original firmware image for your Xperia Play/R800i, you might need to google for that yourself.