I need to replace my 18 hp vanguard engine Model 351776-1046 on my great dane surfer. The problem is I don't know which replacement model to order. There is one model that has a shaft that is 3.19 and one that is 3-5/32. Can anyone help me with this without me having to pull the old engine off before I receive the new one. Also, I was considering putting a 16hp on instead of a 18hp. Someone told me that they were the same engines just labeled with different hp. Can anyone inform me of my options here?