I know this is not a common scenario, but for the few people that upgraded a Mac originally equipped with a Core Duo processor to a Core 2 Duo processor, here is the procedure. I have an iMac version 4,1 that I upgraded years ago, and it worked. The person who found the solution had an upgraded Mac Mini. You will need a second computer that is Lion capable for this tip to work. 
The latest version is 3.2 and is available for Mac OS X 10.8 (Mountain Lion) and 10.9 (Mavericks). If you need the TWAIN SANE Interface for older Mac OS X releases, older versions ar availabe for older Mac OS X releases. Version 3.2 was compiled using libusb version 1.x instead of the old and deprecated version 0.x. Now, drag the System File thats on the drive into the Trash and copy the downloaded System folder onto the root of the drive. Now, by using disk utility 'restore' feature, clone 'InstallESDHacked.DMG' into a flash drive, and use it to install mountain lion on the unsupported Mac. A new procedure has surfaced that may allow owners of the older and unsupported Mac Pro 1,1 and 2,1 machines to run Mountain Lion natively on their systems. When Apple initially released OS X.
As always, Make sure you have a backup of the original install! This worked for me, but always be prepared for the worst.
I made, and used, a Lion Mac OS X Install ESD on a thumb drive. Instructions to make one can be found all over the Mac centric web sites. Then follow this procedure:
- Put the unsupported Mac in Target Disk mode and plug it into a Lion supported Mac.
- Run the Lion installer and make sure to choose your unsupported Mac's hard drive. Let the install finish.
- Boot back into the supported Mac's OS, and put the unsupported Mac back to target disk mode.
- Delete the PlatformSupport.plist at /System/Library/CoreServices on the unsupported Mac's hard drive.
- Reboot the Unsupported Mac and it should boot into Lion normally.
