First thing to do is CHANGE the oil! If your filter is brass, just clean it with kerosene or the like. No need to buy a new filter....if it's anything else, replace it with an OEM filter and be ABSOLUTELY sure it has four holes facing you!
52 on the rear will work well for flat country at reasonable speeds, but if you want to go 60 mph stay with stock.
When you replace the sprocket, get an O-ring or X-ring chain, you won't ever have to adjust it after the first 200 miles.