On an IMOW you can't use a 5 speed tranny, you must use a Transaxle. And, you cannot modify it...
3) DRIVELINE: Must use unmodified transaxle of lawn mower origin, utilizing an open differential single drive belt. The gear ratio from the engine to the rear wheels cannot be greater than 9 to 1 in the highest gear. Example: while in the highest gear the rear wheels can turn no more than one revolution every time the engine rotates nine times.
Thus you have to leave reverse in them in IMOW.