Take it one step at a time:

Check the fuel tank and fuel lines - look for leaks, loose connections, any problems with the clunk, and make sure the tank is mounted right: not too high or low. Try using a syringe or pump to pressurise the tank with a little fuel in there; slosh it around, and if there are leaks, you'll get bubbles.

Recheck your needle valve settings - high and low. On Magnum engines, the idle needle is often a brass screw in the center of the carburetor barrel (the throttle arm side of the carb). Get the manual, or try to download it to see what the initial settings are. Open the needles to the initial settings. If you don't have the manual/can't get it, here are some guesstimates. A good guess for the idle setting is to screw it out until the head is flush/even with the throttle arm. Try starting it, and for most such carbs, you end up adjusting in from there. A high speed needle guesstimate is two and a half turns open. These settings have worked for me when I've had to start secondhand engines but I end up having to dial them in from there.

Make sure you have pressure from your muffler - check the line going from the muffler nipple to the tank.

Check your servo travel - is it reversed? Does wide open throttle mean wide open throttle, and closed throttle mean closed throttle? Use your transmitter's servo reverser switch if needed to set it right.

Check your carb for loose bolts, air leaks around the crankcase, faulty or missing O ring at carb/crankcase, leaks on the fuel inlet nipple. Use thread locker to keep barrel retaining screw from backing out.

Good luck. I've run Magnum engines, and they are great bang for the buck. Nitros are actually a lot of fun, but tuning them can be black magic.