Need help diagnosing Engine issue

[Badapplesrepair]

2009 ninja 250r, engine was running fine but after sitting for a few months it spit all the oil out of the muffler in a matter of minutes. Engine still ran but with very low power. I tested compression and it was very low. It’s been sitting another couple of months and I finally pulled the engine out and opened it up yesterday. I found that there appeared to be a thick mixture of oil in one a few corners that I guess could be from oil and coolant mixing but not 100% on that. There is heavy carbon buildup on both the cylinder heads and on the pistons, rings seem fine, pistons seem fine, head looks ok, I see no cracks anywhere. Could it be a bad seal? Any help is appreciated.