Hustler 66 Inch Super Z Lawnmower Kawasaki Engine Compatible Replacement?

Which Vertical Shaft Kawasaki Manufactured Replacement Engines available today, are compatible with my 66 inch Deck/Cut Hustler Super Z Zero Turn Mower Model#827897 and Serial# 06090302 with Kawasaki Engine#FH721VB00865DS16? Most affordable preferred, yet not to leave me under HP wise. I have the original air filter canister and a muffler if that makes cost difference. Does the length of the shaft have to be precise to what was on the owner. Which of these 3 shaft lengths are compatible(i.e. 3-5/32", 3-15/16", 4-9/32")?