Yes, you can restore to your stock ROM - there are several ways of doing this: after the initial installation of ClockWorkMod (CWM), you can perform a backup of your device. This is like an image that you use to restore your phone to the original ROM/ state. Personally, I deleted my backup as I will never use it again. The other method (and the one I would recommend), would be to download your device's official ROM from sites such as
http://sammobile.com or
http://samsung-updates.com , then use Odin (found here:
http://samsung-updates.com/Odin307.zip ) to flash it to your device.
Cyanogenmod is really clean, and does not contain nearly as much bloatware as original releases. Just looking at the size difference between the compressed CM ROM, and official ROM will give you an indication of how much bloatware is included in the official release. On the S II 9100, the CM 10.1 (Android 4.2.2) ROM is about 180MB (with GApps included, it's about 260MB in total), while the official Samsung ROM (Android 4.1.2) is about 590MB...
If you have any questions regarding requirements, and the flashing process, please do not hesitate to ask.