Like Bubbie said above. Take off the keeper, pull the spring cover up and check that there is an o-ring in there. Spray the spring and inside of the spring cover with penetrating oil. Collapse the tube down from the head and see if there is an o-ring up in the head. Just feel up there with your pinky. You can feel it easily.
Finally, lift the lower tube up out of the lifter cover and see if there is an o-ring down in there. If all the o-rings are in there respective places, then carefully elongate all the tubes to there places being careful not to mess up the o-rings. Reinstall the keeper (depress the spring cover until you can snap in the keeper). Rotate the tube back and forth gently by holding the keeper compressed against the top of the tube and also grabbing the bottom of the tube in your fingers. This is just to try to ensure that everything is well seated. Clean up the area well with soap and water and let it dry.
Check your oil level and take the bike for a ride to see if that has it. If not, and you do NOT have adjustable pushrods, then the choice is yours about whether to live with a little mess, or invest in some adjustable pushrods. Do NOT try to save the one piece pushrods by taking off the rocker boxes to fix this. You can introduce a bunch more problems trying to fix a little oil leak.
TQ