It turns out that many households have the problems and there are lot of helpful videos.

First is what Scott has done in two parts:

This is the part I.
This is the part II, that shows a very good outcome.

My Honda mower stops engage the self-propel capability, this link provides a detail instruction on replacing the driving belt.

Here is the repair steps video.

This guy provides his suggestion.

A detail operation video is here.

Get the clamp fixed

Part on EBay

Part on PartWarehouse

Diagram on Makita parts

This seems to be the correct part

The manual of Makita JR3000V, from the official Makita USA