Update on the HPI gears:

there is good and bad news. The large diff gear is to big for the stock gear case.
The hpi gears 86014 gear diff bevel gears some are useful.
The pack (stock number above at tower) has 4 smaller gears they can be used to replace the stock gears smartech part #11259.
The rest in the pack I haven't ruled out yet but I'll have to spend some time mix matching.
As far as the pinon gear Hpi part number A857 it seems to be almost an exact fit. However I am using a 5x8mm flanged bearing to make it fit better.
Short version :
the 4 smaller gears in the hpi 86014 work and the pinion gear A857 works, as of now that is all that matches up. I am not done though
I have a few ideas and will experiment over the next few days.
Not sure if it helps anyone, you may want to wait until I'm completely done testing the parts before buying any HPI gears.

Have Fun, Enjoy!!!!!