The head gasket is gone and water or coolant has mixed with the engine oil. Replacing the head gasket is no big deal.
Blown head gasket. That milky look is coolant mixed with the oil. STOP driving this vehicle until you have this repaired or you will ruin this engine.
First remove your engine's oil dipstick and look for discolored oil(sometimes milky colored) or if engine is hot small bubbles. Also look for excessive water from tailpipeand/or white smoke.
Any time your engine oil is milky it means you have water in it It's got water in it from somewhere. Probably a blown headgasket. Hopefully not a cracked or warpped head.
A cracked radiator will NOT put water in the engine and make the oil milky. A bad head gasket WILL put water in the engine and make the oil look milky. That does not neccessarily mean you have a bad engine. Have someone fix or replace the radiator and replace the head gasket. It could be a good engine for a long time yet
a small white circle.
Look at the oil on the dipstick. If you have antifreeze leaking into the crankcase the oil will be milky.
Look up a local small engine repair shop in your phone book. They will, without a doubt, have an old string trimmer or chain saw engine that they let you have pretty inexpensively. For "goes fast" stay with a two cycle engine.
Look at the engine on your lawn mower. If the engine has an oil filler tube or a "hole" with a cap on it where oil is added to the engine, it is a 4-cycle (4-stroke) engine. If there is no oil filler tube or place to add oil to the engine, then it is a 2-cycle (2-stroke) engine and the oil has to be added to the fuel.
Look up internal combustion engine - Otto cycle.
most look like a small jet engine
If it is a 2-stroke engine, 2-cycle engine oil is added to the gasoline. If the engine is 4-stroke, regular 30W motor oil is used. To tell if your engine is 2-stroke or 4-stroke, look for a spout where oil is poured into the engine. If there is no spout, then the engine is 2-stroke and 2-cycle oil must be added to the gasoline.
A small engine...it will be in the farthest left gauge.