"there is no simple way to fix this, its either short block or new engine" In 15 years of repairing Briggs engines I would never expect that to be the case. Especially on a 450 series engine- that makes in no more than 3 years old.
Like previously stated there are several reasons for white smoke and all of them are due to the engine burning excess oil.
A. Mower has been stored end up causing oil to enter the combustion chamber
B. Crankcase overfilled with oil
C. Mower has been tipped on its side with exhaust side down whilst also moving the blade manually for cleaning purposes.
D. You are using a 2-stroke (oil/petrol) fuel mix.
pleasant
July 2010