HK motor : Basic line is that you can't expect to much from a $35 motor in a big heavy Savage Flux.
CC motor : Guarantee or not, still a awesome price :)
I don't believe a 1518, let alone a 1520, will fit a stock Flux based on this picture (I took a long go back, before I went XL).
But I'm not sure!!
The 1512 is a 1/8 buggy motor and it's not advised to use it in heavier trucks. The 2350kv is an economical option on 4S.
ps. don't go messing with dual motors.