I have used snap ring pliers with the removable tips which are thin enough to get in the slot.
The replacement brass rings are available at several aftermarket sources.
However you may be wasting your time and money trying to replace them.
If you look closely the brass ring is staked from the inside of the synchro drum which causes some of the
brass to protrude into the small holes in the housing. That is essentially what holds the ring in place.
I'm sure that there was some special tooling designed for this process.
Just pressing in the rings won't hold them in place long. You may be better off trying to find a better one.
Fred
