Start by tuning your Idle needle so there is about 1mm gap when looking down your carb. After a little break in you can then adjust accordingly while doing the rest of your needle tuning.
Perhaps you have an air leak. You can pull your fuel tank and fuel lines out and stick them under water, then blow through them. If you see air bubbles, you've got a leak. You can also try sealing your engine, THIS thread will instruct you for that.
What exactly are you looking for? RTF? BNF? Kit? Brushed? Brushless? What kind of flying do you want to do and where?
Hopefully others can help you more. I've only ever flown cheap RTF drones.
It is this assembly. You tighten the nut under the spring.
But if you pick it up and it still doesn't want to turn, you likely either stripped the servo horn/ gear teeth or damaged something inside the servo.
You're right, not terribly hard to buy a cheap used flux. Drive it stock a little bit then maybe merge the two chassis. Might come out cheaper than converting your nitro truck.
That's a K4.6, which only came on the Savage X SS and Savage SS. If you post more pictures of the truck we can confirm what you have.
https://www.hpiracing.com/en/part/15201
There is an exhaust that mounts behind the rear shock tower, in the position you speak of. It looks great! It's one major flaw......you hit the truck on its tail and you destroy it. I suspect the same would be true for a tank sitting back there.
The most common relocation is what is called a mid-tank mod. The tank is moved to the area of the electronics box, usually getting a bigger better tank as well. This also requires purchasing a small receiver box and relocation the receiver to the outside of the TVP.
I keep almost getting pulled in with that sort of thinking. Truth is, if you don't have a worthwhile 1/8 combo (esc, motor) and batteries/charger you could still spend several hundred dollars more.
That's when I think about just buying a new/used Flux and keeping my Savage a nitro.
Then I...
Post up some pictures. We can likely identify most stuff.
When you say stock, are you asking for the literal stock exhaust for your truck? Or any stock Savage exhaust?
There will likely be a bit of debate on this between people. I'm of the opinion that it is always an easy bit of insurance to seal the engine. That, as well as putting some green slime on the o-rings, loctite on the screws into metal and generally going through the engine to make sure there...