It won't hurt to run a little more, after dealing with the oil leaks. If the rings are completely worn out, or stuck so tight into the grooves they can't seal against the cylinder walls, the smoke won't get any better. If they're just stuck and the solvent can loosen them up the smoke might dissipate after some more running. Either way, all you're out of pocket so far is a little oil and diesel fuel and some time.

Fix the valve cover leak, warm it up one more time with the solvent onboard, drain it hot, and refill with 30 or 40 weight straight weight oil. If there's an oil filter, also change the element and suction the leftover 1/2 quart of oil out of the canister. It will either improve enough to do a little driving, or at least you've done a pretty good sludge cleanup with the solvent prior to a teardeown and rebuild.
Jerry



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