Re: I'm Restoring a late 40's Rocket - Then comes the motor....
That is a great looking bike. Perfect for a motor. It would be awesome to use an American engine of similar vintage. A cast iron Briggs may be a little heavy, but I've done quite a few with aluminum 5hp B&S motors. They don't come in a kit, but perform and feel great. Check my threads for photos if you're interested.