Cranky ol' bastid that I am I see little reason to stay within the constraints of such a meaningless law - they needed a number and the bureaucrats picked one.
However, weekend fun is a minor and as such is unfortunately all the more likely to be targeted by hostile enforcers whom have little regard for the letter of the law - let alone deviations from it. There's little difference between a hopped up 49cc and a stock 66, so why not actually be legal in this case? That way he's covered by the plate on the side of the engine and can provide receipts in court if need be.
I understand my own hypocrisy - havin' an oversize engine yet suggesting legality... *shrug* I've made my choice and I understand the repercussions, as does weekend fun I bet
Cops just don't listen to kids - even when they're tellin' the truth. Best to cover the bases I figure.