"Wear a helmet, farangs are normally not stopped, insure it, and get a fake plate"
Yeah, people have gotten away with a lot - in Northern Thailand. But now, you are not just driving an unregistered bike, you are driving an unregistered bike with counterfiet documents. The police really don't like that.
Also, with a fake plate you can't get the road tax sticker required for the bike. The police tend to notice things like that.
By the way, both the road tax sticker and the insurance sticker are issued with the matching plate number printed on the sticker. So you can't use one off a different bike.
Unregistered bikes are usually that way because of the cost. You save money up front, but you cannot leave the country, and will have trouble if you leave the North.
BobS
"The true secret of giving advice is, after you have honestly given it, to be perfectly indifferent whether it is taken or not and never persist in trying to set people right."