You can clean out totally the entire head oilways, with the head in situ, a bit cramped to do but no bigee.
The heads oil gallerys run on the outside/side of the head each side, and are plugged at each end, the oil drain back is easy to block with paper towel etc, and use a drill(in your hand) to check each oilway is clear, can take the plugs out each end and pull through with proper lace brushes to make sure everything is 100% spotless.
tabetha