A recent 49cc I bought didn't even have the jug machined at all
I did the sand paper/glass thing to have a decent surface for the gasket to seal. I also use Permatex Copper Spray-a-Gasket. The jug-to-case gasket I put silicone gasket on to help it seal. The stock head gasket is made from dead soft aluminum. It's made to crush to form a seal. The after market gaskets are a bit thicker and not as soft and seem to seal better. Another thing to watch for is the Chinese sometimes don't tap the stud holes to the bottom. Not having enough thread in the block can cause the threads to pull right out of the block. I tap the holes. You can get a good Metric tap set from Harbor Freight for under $15.
Use a pattern of some kind when you torque the head down, something like an "X" or "Z" pattern so the head pulls down evenly. Go up in torque in stages like 24 inch pounds or 2 foot pounds. Believe it or not I use Moly assembly grease on the nuts for the head studs. I learned that from ARP, American Racing Products. They make bolts and studs for race cars that are considered the best you can get and they say to do it that way, so I do
Just going around the head and torquing to spec on each bolt in turn will almost guarantee a leak. Re-toruqe right after the first ride after letting it cool. Don't torque hot, readings can vary a lot from the cold torques. Check it again after every tank of gas a couple of more times and you should be good.