Assuming you get the bolt out, it's probably an M10 bolt available from anywhere. SOME (a very few) Honda engines have e 3/8" UNF bolt (as fitted to Briggs/Tecumseh/anything except Honda). Just get one that engages a MINIMUM of 40MM of thread in the crank when tightened.
phil_saunders(a)bigfoot.com
Phil Saunders, May 2011
