You need to contact the vendor to see if they will provide 64 bit media to reinstall.
If they refuse, ask them if you are still entitled to either 32/64 as the EULA states.
If so, you can locate your own 64 bit copy of Windows 7.
All versions are unlocked in the installer by removing the ei.cfg file in Sources, then recompiling ISO using Vlite to burn to DVD.
You may want to make a Windows 7 backup image of your current install to fall back on if for some reason the reinstall won't activate afterwards at Computer>Properties link. It may require a robocall to MS to trade numbers to deactivate the old installation before activating the new one, although using the same hardware may not require this.