Hi jroyse,
While it can be a challenge, it is possible to make the manual clutch work on the new edition Whizzers.
The majority of the problem is caused by mis-understandings. The object is to make the "clutch" work correctly by first adjusting the front belt. It will help if I explain the differences between the vintage Whizzer clutch and the new edition version. The vintage system had a special clutch handle and had a longer throw, meaning it pulled the clutch arm a longer distance, secondly the clutch arm attached to the motor with pin not a bolt for a much tighter fit. To add to the problem the clutch arm wasn't the correct length causing the front belt to rub the bottom of the belt guard mounting post, and of course using a FHP belt around small pulleys was another minor issue. The FHP belt runs hot and often trys to enguage the clutch at idle when stopped.
Sounds like you already obtained the correct front belt [15285] and has notches to work around smaller pulleys.
I would suggest you remove the rear belt from the sheave, loosen all the motor mount bolts, move the motor as far to the rear as possible and determine what needs to be done to make the front belt tighter. If the spring isn't pulling the arm high enough to pull the belt tight, shorten the spring. Don't forget to loosen the clutch cable adjustment to allow the arm to move as high as possible. Next I would tighten the bottom motor mount bolt, place a wedge [use wood] between the top rear of the motor and force the motor forward untill the belt is "snug", not extra tight. Next check and adjust the clutch cable to make sure the belt loosens enough to release the front belt when stopped, and must be set with the metal belt guide installed [was part of the belt guard on early versions]. When satisfied it is working close to correct, tighten all the motor mount bolts. Install the rear belt on the sheave and make sure it doen't pull the clutch downward in the process.
The AX numbers on the rear belt doesn't translate to actual length. An AX 62 is 64" long, AX 61 is 63" long, etc. Always use AX or automotive wedge belts because of the small pulleys.
You are welcome to contact me direct and I can "talk" you through the process.
Have fun,