I'd be surprised if this were anything but a standard 1 1/8 headset (34mm headtube ID). Pull the cups and measure. There's a jillion of them out the that will fit the bill- anywhere from $25 and up. Stack height won't much matter cuz you have enough spacer on there that can be adjusted. You'll likely need to replace the crown race, or race ring, whichever is appropriate to the new headset.
OTOH if it's still smooth when turning, why bother? They (the bearings) support some weight, but they don't see any RPM. Grease it and go.