Craig, If it were me I'd pull the valve cover and assess the situation. If its so nasty I'd clean out what I could making sure oil is getting to the rockers and put the motor on a real frequent oil change interval. The addition of Marvel can be good and won't hurt.
As far as any cleaning solution I've always remembered how much effort it takes to clean parts in a parts washer with a brush and think about some solution being able to clean the motor inside.
I had a SB chevy motor once that was so nasty oil could not drain back from the heads so I cleaned out the drain holes and changed the oil a lot and was surprised at how well oil will clean things up over time.
Even if there was a solution that could dislodge the crud would you want it running around in the motor.
