I'd have to count them but, 43 teeth sounds right. As I recall the original pitch ball diff spur gears came in 41, 43 and 44 so, 43 is probably the stock diff.
I prefer the disc dampers but, an oil top shock is much more adjustable than the stock friction top shock. You can run both if you want. I use silicon oil of different weights on the disc damper.
Not sure if this is happening to you but, the front tires have a tendency to roll over themselves on high speed corners, this will make the rear hop. Tire tape or gluing the front tires on the rims will stop this.

twisted concept, AM or ASF module? Both are hard to find now. I suggest the mini z racer forum, maybe you can get a used one.