Could be valve stem seals but most likely is a HG. If its still running and you are good with tools and have the slightly more advanced stuff like torque wrenches/claw pullers etc... you can DIY. The OHC engine is not some sort of rubix cube vs a pushrod like everyone seems to think. You have about a mile of chains in the front and the cams are in the heads but its not super hard.
With that said, I'd rather do 3 pushrod headgaskets than a single 4.6L lol.
2000 Mustang GT Steeda build #48. Bolt ons, suspension, Novi 2000.
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.