Looking good!
The oil leak that is the most common is leaking at the head gasket, there is an o-ring on one of the head dowels that is the oil supply up to the head. The telltale sign is that dark ring of cooked oil where the head meets the cylinder. It takes pulling the motor out of the frame and tearing down the top end to fix it, so if its not leaking badly i would just keep an eye on the oil level when using it, provided its not burning oil and smoking badly or otherwise has other problems that would benefit from a top end refresh.
I have a chinese ebay carb on mine to, runs great, i kept the needle that came with the chinese carb but swapped the jets over from my stock carb, that seemed to be the right combo for me.
When you get the DG exhaust on it you might have to turn the mixture screw and richen it up just a smidge, but ride it for a bit and check the plug and see if its running lean or not. Mine only needed a slight tweek to get the plug a nice even tan/brown color. Reading the plug is the best way to find out whats going on. A lot of guys said it would take a jet change after putting on the less restrictive exhaust but i didn't end up doing that on mine.
---------------------------------------------------------
1984 Honda ATC200ES "Big Red"
1982 ATC200E "Hondie"
1988 TRX300FW "Project Quad" Still in progress....