I don’t think you’re likely to do any harm by scrubbing with some sort of petroleum based solvent. Just make sure the crankcase gets drained afterwards. If I was going after that kind of stuff, I’d probably use gasoline or diesel. Just don’t get brush bristles into the bottom end.
Is the engine a runner? I have successfully cleaned sludge with a procedure recommended by some here. Involves running the engine at a fast idle with a crankcase that is full of oil, and then adding a couple quarts of diesel. If interested, can provide some more detail.
