yes 5-40 both above and below that thread into the engine mounts, i would get washers for above and below, but the most necessary ones are the ones below.
power switch, i would either get a new switch with the right connectors, or what i did was just solder new connectors to the existing switch. either way works, just know that you will have to kind of cut into the RX box just a bit for the connector to get through it either way, as the servos are male connectors, the switch is gonna have a male connector so a bit larger.
i would definitely be questioning all the servos being dead, you sure you had them plugged in the correct orientation? black to the outside, white or orange to the inside? mine are all still bone stock that it came out of the box with and still work flawlessly.
i would guess that they are the stock gear ratios, usually if they were not people selling it would know. i would count teeth on the CB just to make sure. the spurs should have tooth count stamped on them.
most parts are going to be available through Amain and Horizon, many parts are even still compatible with the LST XXL/2 and 3xle.
truck looks very awesome man!, very clean. doesnt look abused lol looks like you got lucky and they upgraded the plastic diff case with an aluminum one. one of the best upgrades!