You can get the key from some program floating around, forgot what it was called specifically, but you plug in all of the crazy lite-on barcodes and it calculates your drivekey, alternativly you can e-mail the information to Foundmy and they'll do it for you. I'll dig around my large amount of harddrive space to see if I can find that program and PM you what it is when I find it.
To answer your question, there is no soft method to get the key off of a lite-on, and you cannot replace the drive without the drive key (unless you were to swap the circuit boards).
[Only Registered Users Can See Links. Click Here To Register] is a guide on building your own probe to extract a key if you're at all familiar with electronics. I'm really hoping that you didn't erase the drive without extracting a key, since there is no way to go back on a lite-on erasing it.
------Update------
So, I found what I was thinking of. Turns out it wasn't a drive key calculator, but it fixes bad serial data from key dumps. JungleFlasher does this for free, where as Foundmy charges about $35. You pretty much have to dump the key data if you want to replace the drive, other wise it will not work. And do not erase the drive data until you've dumped the key successfully.