Your transmission is the 2.5 style with the single speed conversion. The primary output (middle shaft smaller gear) and the forward only gear (bottom shaft) are stripped.
Unfortunately, there are no steel gears to replace that setup.
You would need to use RRP gears and make a locking plate for the standard 2nd gear to be able to use reverse and have brakes.
The single speed conversion you have also is an in-between gear ratio compared to the stock 1st and 2nd gears. Using a stock ratio 2nd gear locked will result in a taller final drive ratio compared to what you now have/had.
The single speed conversion was never intended to be run with a brushed setup much less a brushless system and will fail miserably with either.
Unless you are willing to drop the coin on RRP gears and then do the needed modifications to have reverse and braking with the motor/esc you would be better off installing the BL Emaxx transmission.