Re: 49cc engines require driver's license
You can get inexpensive metal stamping kits at harbor freight. You can stamp "48cc" or less on your engine somewhere.
I don't know of an easy way of reducing engine displacement. I haven't picked up a Hotrod magazine in a while, but I believe you would need a de-stroked crankshaft or cylinder sleeve with a smaller piston. The stock car racers must have some tricks for accomplishing that.
“You're only young once, but you can be immature as long as you like" -Unknown