Well... either someone changed the hood, or decals, or motor.... or something because none of the 12XX anything had a Mag 18. Now you could have an 1812. It would have a Mag 18 in it. Better post a pic and the numbers on the serial tag.
As far as the engine goes, if you had gas in the oil, you likely need more than a needle valve and carb rebuild. Flush the entire fuel system and replace the lines. REPLACE THE FUEL PUMP. It is likely what is leaking. Very common.
If you had loose head bolts, tightening them is a waste of time. Pull BOTH heads and plane them and replace the head gaskets. Run the engine and retorque. Run it about 25 hours (work it, not just idling) and see if the smoke stops. It's likely someone was running it with gas in the oil and the rings aren't seated anymore. They will likely reseat themselves. I've seen Mag 18's that didn't make it to 1000 hours, and others that ran 2X that. So... depends on how it was treated as to how long it will last. Just like any other engine.
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