No idea on the oil leak, but the surge is due to a plugged/gummed carb jet or fuel delivery concern. Pull the carb bowl off, remove and clean the jet, reassemble. Verify you have proper fuel flow at the carb fuel inlet. Easy job.

Go to an auto parts store-buy several cans of Gunk engine cleaner and a parts washing brush. (Get the regular Gunk! The foaming stuff is not so good.) The brush will have short stiff bristles. A regular paint brush won't do the job.
Then clean off the heavy grease and goop, then spray the engine cleaner "Gunk" all over the engine and its parts. Allow to set for 5-10 minutes, then using a water hose, give it a good bath, until it comes clean.(Do not use a high pressure machine that folks use to clean walks, and other stuff. It will insert water where you don't need it!
Do this on a hot sunny day! After you get the whole thing clean and sprayed with the water hose, then move it into the hot sunshine, and allow it to sit and dry up. I do this to all of my stuff, and it doesn't seem to hurt it.
Some who like to argue with this old bird, will take me to task for the preceding instructions, but its my way, and i ain't changing! And, WD 40 is NOT an engine cleaner!
If your engine is Over-head valve, the valve cover gasket is leaking. And, the engine surging is caused by dirt in the inner passages of the carb. It needs a good soak in carb cleaner, and a complete disassembly for cleaning. Hope this helps some. Rusty Jones