Thanks to everyone for the input.

Regarding my statement:

"My drive belt was worn to the point that it was seating all the way in the bottom of the drive pulley and slipping badly."

After purchasing the new belt I checked it in the drive pulley and found that it also went to the bottom of the pulley.  I considered moving the fixed half of the pulley over on the bearing tube but did not.

After the reassembly and a test ride, I found that the new belt slips as badly as the old one.  While writing this it dawned on me that I have my wheel back far enough that the tension idler is not at the ideal position shown in the manual.  Maybe a first step will be to readjust the wheel to correct this and see if the additional if minimal wrap on the drive pulley has an effect on the slippage.

If the wheel adjustment doesn't make enough difference, I will remove the clutch / transmission and move the pulley enough to hold the belt up on the sides.  Wayne said that the pulley would have to be heated to be able to move it.

Has anyone else already made a similar adjustment to the fixed pulley position?

Picture of the rear brake showing the lever and rod.  The rod is currently just stuck into its connection.  I am in the process of reassembling the rear wheel, fender, cargo rack, etc after removing to install a new drive belt.

Wow.  What a coincident.  I am in Georgia but the bike was originally titled in Ohio and then went to Illinois.
I am attaching a picture of the original title.

My motorcycle is a 1958.  I was having some issues with belt slippage on the belt to the rear wheel.

Just want to share some information that I have learned while going through my clutch / transmission unit.  Mine has 2 bearings in it both being 6300 radial ball bearings available from any bearing supplier.  They are available from McMaster-Carr under part number 6661K121.  The idler pulley is machined aluminum and uses 1 each of the same bearing.  All three bearings had issues but the one in the idler was almost seized up.

My drive belt was worn to the point that it was seating all the way in the bottom of the drive pulley and slipping badly.

A suitable replacement for the long drive belt I have identified as a BX75, Gates number 90136075, also available from McMaster-Carr under their part number 6057K147.

I have not actually installed these items yet but will follow up to confirm that the belt is a fit.

What I need to know is has anyone identified the primary drive belt?  Mine appears to be the original with the original number.  I would prefer to have a new belt in hand prior to removing mine if possible.

Thanks.  Can you give me Wayne's email address?

I have a 1958 Simplex with a non-working throttle.  I removed the grip and internal mechanism and found the cable is damaged.  It is similar in appearance and resistance to movement to a mower control cable, not free moving like the cable on my other motorcycle throttle.  Is this a design feature to allow throttle position to be maintained without holding it or should it return to the closed position when released?

Any input welcome.