Some of the new EPA friendly 50cc have a slant plug head, some have old round heads, some have square, most larger than 50cc have a larger squarer head.
Many experienced MB'ers dont even know the diference how do you expect a cop to know? Theres no way they could know unless they pull the head and do a volume test and that ain't gonna happen. Been ridin for 3 years never been stopped.
If your really worried get an actual 50cc engine or get a metal stamping kit from Harbor freight and stamp it as 50.