I'm rather shocked that this has not been answered since the solution is very simple.
As mentioned in previous posts, you would not want to convert it using C#, but just once. This is easy to do with PuTTYGen.
- Download your .pem from AWS
- Open PuTTYgen
- Click "Load" on the right side about 3/4 down
- Set the file type to *.*
- Browse to, and Open your .pem file
- PuTTY will auto-detect everything it needs, and you just need to click "Save private key" and you can save your ppk key for use with PuTTY
Enjoy!