Outdoor Power Equipment and Small Engines - Engine blows oil out muffler & dipstick tube

My grandmas neighbor gave me a push mower thats in great shape. It has a 6.5hp briggs on it but it wont run good. I checked the oil yesterday and it was fine. I ran the engine and it wont run at high rpms and oil blows out the exhaust and I unscrewed the dipstick and oil comes splattering out while it was running. I pulled the head off yesterday and everything looks fine. I thought maybe it had a cracked piston or something.

Anybody have any idea on whats wrong?

If it is blowing oil out the exhaust and running poorly, it is not "in great shape". You should never open the oil filler when and engine is running. It will blow oil out! When you had the head off, did you inspect the cylinder wall? Was it scored and scratched? Did you check the compression with a gauge? What makes you think the piston may be cracked, did you see something amiss on the top of it? I would guess that a worn or broken oil ring is more likely than a cracked piston. A little more info would be helpful.

The head is still off, the cylinder looks fine, lots of carbon buildup on the head. The head gasket disintegrated when I pulled it off. The mower looks in great shape, as far as the engine I have no idea. They got a new one, they didnt say why. It doesnt look very old so I didnt think it would be wore out. My grandma lives in a trailer park so this probably cut 100sqft of grass once a week.

Sounds like the oil is too full?

Change the oil and fill to proper level.
Check to make sure the choke is not staing close or partially closed.
Possible rebuild of the carb.
Change the spark plug.
Make sure gas is fresh and not been in tank for a while.

I overlooked where you said you checked the oil and it was fine. Is this an OHV engine? If so, it could be a blown head gasket between the cylinder and push-rod chamber.