I believe the old style drive temperature sensor is available for purchase from various sites around the web. I have an early 2008 iMac that has the old style sensor, so I didn't have to deal with this particular issue.
However, if you can solve the temperature sensor issue, I found that mounting a 2.5" SSD into the iMac with the [http://www.newertech.com/products/adaptadrive.php|Newer Tech AdaptaDrive] 3.5"–2.5" adapter worked flawlessly.