Since it sounds like you will not be removing the heads, you're gonna have to be very careful about what junk you dislodge as some of that nasty stuff will end up in places you don't want it. The oil drain-back holes are gonna let the junk flow down into the bottom end...but not necessarily all the way into the oil where it can be drained out...some may catch on ledges, corners or hollows. That's the problem...easy to clean the head with brake cleaner etc, but hard to get the residue outta the engine. However, if you can't stand the thought of replacing the engine without cleaning the top end, you might want to spray the hell out of it with brake cleaner, catching as much as you can with rags, then spin the oil pump drive with a drill motor to let the oil flush the gunk down into the pan.
The other thing you might do is drain the oil, then invert the engine (assuming it's on a proper engine assembly stand). Now, you can spray your solvent UP towards the heads and let the gunk fall out of the engine away from the bottom end. As you might imagine, this can be/is quite messy, but should work just fine.