I'm not familiar with the engine on your machine as to whether it's 2 or 4 stroke or air or water cooled, so I'll make some generalities.
As to the oil leak, well, it leaks. You'll have to find where it's coming from before anyone can tell you how to fix it. First, wash the engine down good to get rid of all the old crud and oil, run it and then park it over a piece of cardboard. You will be able to use the drip spots to narrow down the problem area. Remember that where the oil if dripping off may not be where the actual leak is, hence the reason for washing the engine.
As to the smoke, where is it coming from? Out of the exhaust pipe, from the air breather or the tube that goes from the crankcase to the breather or is the oil leak dripping oil onto the exhaust? is the smoke bluish or whitish colored? The former is oil and the latter coolant if the engine is water cooled. Does it smoke worst immediately upon start up and then get a bit better or does it get worse as the engine heats up? When you unscrew the oil filler cap (assuming the engine oil goes in the crankcase and not in a separate reservoir) with the engine running, does oil fumes pour out the hole?
You need to provide more detailed information before anyone can help.
