A gearbox with a 23t pinion would be a 21-22-23 7R which will accept a 22t aftermarket pinion. If you are looking to replace the entire gearbox than you can use this gearbox: http://www.mltoys.com/Power-Wheels-Must ... 8-2821.htm
. Or if you have just broken a gear you can use the same gear (1st,2nd,3rd,final) from any other 7R gearbox to replace it. Don't get lost in the part numbers, there are 3 7R gearboxes, 15-16-17, 18-19-20, 21-22-23. You can interchange any gear from any 7R to fix a broken gear but you must match the motor pinion with the gearbox. For example you can't put a 22t pinion in a 16t gearbox.
Regarding the first metal gear it is my experience that I have never broken the first gear. Here is a recent comment from the MPW facebook group which might be helpful
" From my experience and observations it seems to me there are two forces entering the gearbox, one from the motor, the other from the wheels. I use 300w 1,300mNm stall 775 motors with an 1kw ESC and overload it to 250+ lbs and have never broken any gear on a 55+ amp hard [ESC] launch. In other words, I have torture tested stock gearboxes and have never broken any gear on flat ground, especially not one on the motor side of the gearbox. Regarding the other force introduced from the wheels, I have noticed gears break [for us] over major changes in terrain on our property. There is a short but steep valley at the bottom of which we broke two of the larger gears, a drop off at the driveway which has broken a [middle] gear and a section that has ruts which has also caused breakage. We have not broken a gear on any flat portions of the lawn. This has lead me to conclude that significant changes in momentum transferred through the wheels into the gearbox (which can be relatively large considered cars can be traveling around 10mph while carrying 100+ lbs) are much more likely to break gears than forces introduced by the strongest 775 motors, using the strongest batteries (lipos) using a 1,000w controller which can peak at 55 amps throughput. Also, in my 5+ years of breaking cars, I have only damaged one [first] gear on the motor end of the gearbox which melted on the post while using HPI 550 motors (very fast motors). Once this gear was replaced and no other changes were made for some reason the replacement gear never failed under the same circumstances"