Welcome! I know the feeling about having to build one of these! I stumbled on this site while looking up info on shoehorning an electric scooter motor into my son's Power Wheels, and a few months later I have just finished my second build, lol.
That being said, I think I followed a similar path that you are on. My first build was a 30cc Ryobi 2 stroke (from a pole saw, but the same as the 30cc on the string trimmer) as a friction drive. Even with access to only hand tools, it was a pretty easy build, and fun! Cruised pretty easily around 20 MPH, topped out at 24 but was very, VERY buzzy. BUT, since the motor was free and I already had the bike, it was a cheap way to get my feet wet. The build thread is here: Mongoose XR-75 friction drive
Then I sold it to a guy nearby on Craigslist to fund my 66cc China Girl build. My motor had no outer markings or labels so it looks identical to the 48cc motors, but if you want to stay legal you might stick with the smaller one. From what I hear, the power is not all that different. Since the China Girl comes as a kit, unless you want to do something custom and out of the ordinary, the installation is pretty straightforward with not much custom work needed. Here is the build thread for that one: Yep, another Cranny
Good luck, and as Venice told me, welcome to the addiction!