At this point, I would be working kerosene in around the guides, not trying oil. The gum in the guides is the most likely issue and it'll be hard to work out. I would not hesitate to use something more severe at this point, like even electric motor cleaner in small amounts in the guides (which will dissolve petroleum based gums like you would not believe...along with paint, etc.); there is not much to hurt except guide seals and they probably need to be replaced anyway, and you can squirt in oil/kero when they free up. Whatever goes in will/can be diluted.
Do you have a lever-type valve compression tool for a shaft type rocker engine? If so, I'd pull the springs off those valves and work them with cleaner by hand, with each piston at its TDC area. (Or use the rope in the cylinder trick.) It'll clean up a lot faster. I would not hesitate to do each cylinder one at a time, that way.
With the rust in the head area, there is a possibility that there is rust on the valve face and seat for any valve that was open and moist air reached. If the valves are free, that may or may not work out.
If you think the valves are freed, and the engine runs at all then change the oil and filter, warm it up, put in a cup of ATF and cup of Kero (homemade motor flush) and run for 15-20 minutes and then drain the oil and replace that and the filter again. Oil is cheap, relatively speaking....
Have you dumped/cleaned the fuel tank?
BTW, the oil on the rockers ought to be dribbling from a small hole in the upper end of each groove or slot on the top of the rocker. If you get this to run, it will be fairly obvious.
