Flushing a engine block that has been sitting.

[69PACE]

I've never done this myself, but an old farm trick is to use straight white vinegar to dissolve blockage. You will have to drain, flush then fill, but if the plug is organic mater it will eat its way through in a couple of days. I've used it on drains and the pipes are fine.

Other then that I'd say the advice to strip her down, and caustic boil is the best way to make sure everything is open. Or use pressure washer.