The crank seal problem isn't specific to Invaders or Kawasaki's, any 30 year old sled will need the crank seals replaced. The seals are rubber and fail with age, when they fail, it can cause major engine damage. Invaders are fast for an old 440, but the 700 you were riding is faster. Invaders can be nice reliable sleds, but most I see need ALOT of maintenance work before they are reliable. New fuel lines, pull cord rope/rebuild, carb cleaning, crank seals etc etc etc. But they are a great sled! Good luck!