If you have to dig the head up to fix it (which you do), I would just replace the head.
Its the riser/wiper seal thats worn out. Just go get a new one. It ain't worth replace the seal.
Ahh so the seal. A new sprinkler head is $27. The seal, if avail at Home Depot, i cant imagine it being more than a few cents but i've never looked at such things before. Can the seal be bought separately at any hardware store?