There are as many answers to this question as there are members of this site.
Are you looking for an in frame type or above the rear wheel type?
In frame: EZ MOTORBIKE COMPANY
Also: 50cc Complete Gas Powered Engine Kit (Honda) - $499.99
Above the rear wheel: Complete Rear Mount Engine Kit (Honda) - $569.99
Also: Motorized Bicycles Detail Page
Also: Golden Eagle Bike Engines, Geared Belt Drive Bicycle Engines / Motors for Mountain Bikes & More
These are the most common ones I have found. All are of good quality and value.
Personal opinions vary depending on what one expects out of a kit.
Hang out for awhile, read up on your options. There are many more I have not listed.
Why don't you introduce yourself in the Introduce Yourself section?
Introduce Yourself - Motorized Bicycle Welcome Forum at Motorized Bicycle Forum
BTW, Welcome to the forum!