A leak down test will indicate bad main seals, bad o-ring head seal, anywhere bolt holes go into bottom end, leaking rotary valve cover etc. If it was a carb problem and only 1 carb the plugs should look close to the same unless 1 inner main was leaking. A weak spark from high resistance connection can can cause incomplete combustion when you nail the throttle, have you trimmed the plug wire ends at the plug cap? When it's smoking does the smoke look black or have a blue or white/light gray tint? My plugs look like yours all the time on purpose but probably not for the same reason. To eliminate the oil pump plug the line and premix once to be sure. Not smokey when you remove the seat is it? And if the little plastic check valve that's screwed onto the fuel block that's under the lever leaks the carb will dribble fuel down the throat, use a mirror (so no burnt eyebrows) and look down the carb throat while it idles and see if it's dripping fuel. One that leaks will load up bad when you chop power after a full throttle to high RPM section. If the prior owner fooled around maybe he removed or altered the restriction in the return fuel line at the carb. I've seen skis run funky from the water lines incorrectly routed. Don't give up, you'll figure it out.