Welcome to the forum. We're glad you've found us and hope we can get you started on your first motorized bicycle project.
From the images it appears that your choice of the Trek 990 will lend itself well to motorizing. The frame looks to be a standard 'V' style which is a plus when it comes to mounting the engine. http://www.bing.com/images/search?q=...24&FORM=IQFRBA
You'll have others reply and offer suggestions as to their choice of vendors who they will recommend for your first kit. There are many to choose from.
Good luck and have fun.