On the laws thing, many states differ so would check local DMV. Some states including my own allow for up to 5 HP and many only up to 50cc. Some require registration while most don't. Just a thought.
The largest engine I have mounted in frame was a Harbor freight 79cc. There are commercially available mounts for them that are bolt on. I made my own with a drill press and some angle-stock. For first builds, I think it would be easier to go with some thing already made and work up to a DIY build. Again, just a thought.
No matter what you decide on, these things are great fun and you will enjoy it.
Welcome to the forum!
I got curious and looked up motorized bicycles on the VA DMV site. All the pages I found on " power-assisted bicycle"s seem to have been taken down. Commonwealth of Virginia Department of Motor Vehicles
But Mopeds had this; Commonwealth of Virginia Department of Motor Vehicles
The laws section here has this; Google Custom Search
But be sure to double check the information you find is up to date.
But honestly, I have never been bothered about engine size.
Lots of great folks here who enjoy helping so ask any questions you have. Can almost guarantee some one will know or have tried what your thinking of doing.
Enjoy and again, welcome. Post lots of pics!