I've build some with the Grubee engines and some without. I kinda think the quality is better with the Grubee motors but it's hard to say that with 100% certainty. If the price is the same or close, I'd go that route.
Most states require the motors be 50cc or less to be classified as "scooters" and therefore, street legal without registration, but it's hard to tell a 50cc motor from a 66cc motor so most people opt for the larger cans. Generally speaking if you go under 30mph and don't make a nuisance of yourself the cops probably won't give you more than an amused smile.