it is a caldera 10e
with stampede wheels

the pinion is a different pitch. The stock pinion gear size will work OK with the traxxas motor. The stock motor is 3750kv...the vxl motor is 3500kv. so it will run a bit slower with the stock gearing and vxl motor. I think the gear pitch is .08 mod.
Parts can be found on ebay, redcat's site, and a few online dealers.
there are similar models, the XB and the SC 10 E (buggy and sct truck) and all parts are the same except pinion gear, tires, body, body posts.

1 word of caution, the stock brushless system's esc has punch control. this is a good thing as the truck has no slipper clutch. If you install a vxl system and use a 2s or 3s lipo and have a "heavy throttle finger" this will result in damaged gears since the vxl speed control has no punch control. If your son has good throttle control, it wont be an issue. but if he is used to punching the throttle, then it will strip gears.
Using a ezrun 60amp speed control (or other with adjustable features like punch control) with the vxl motor will prevent that possible problem.