I bought a champion generator 2 yrs ago.It ran when I bought it. Have not occasion to use it until now. It will not fire. It had some rust on the flywheel. Removed the rust. I replaced the spark plug.

probably==varnish from gas sitting too long or fuel filter.
I was able to get past this on one of my machines by using starter fluid and keeping it running for a minute or more until it cleaned itself. YMMV. Otherwise, it is a disassemble/clean project.

You said you have fuel but do you have fire? It doesn't matter how long it cranks, do you have fire at the spark plug or not? Gotta have both.

I can not answer the question about the internal module battery. But wanted to suggest that when you changed the crank sensor you might want to lightly clean off the rust on the crankshaft pulley damper. The pickup sensor for the crankshaft needs to detect the magnet in the damper to fire the pulse. You have 10 years of rust build-up on that part.

I learned of this condition through working with old lawmmowers and tillers. In the off season the crank damper would rust and the engines would not fire. Sanded the parts and instantly there was spark to the system,

One of the power wire going from your ecm to a sensor or solenoid is shorting to ground. Get a wiring diagram and an ohm meter. disconnect the battery and start checking each wire measuring from the wire to ground. as you wiggle you will see the meter show 0 ohms when it is shorted. These are the hardest and most time consuming kind of electrical problems .good luck

Try cleaning the carb out,(hog wash carb cleaner I use) the float may be stuck or the needle & seat.You may need a rebuild kit for the carb,about 12 bucks.hope this helps

check the on/off/reserve valve

With the engine off, check the exciter brushes and slip rings which supply magnetic power for the alternator winding in the stator. Also, check the motor-to-alternator mechanical coupling. Check the breakers for tripping. To service beyond this point requires a schematic of the alternator, a voltmeter, plus knowledge of what the schematic is telling you. Good luck!

youll have to see if your coil is firing

I assume it is a hand start genset. The most common problem if it has been sat for a while is rust on the ignition timing flywheel. If you take the pullcord housing off (recoil) then you will see a big flywheel with what can only be described as a big forked prong with a couple of wires on it. This flywheel should be stainless steel and rusts when sat around for a while without use. Its job is to complete the circuit to the spark plug every time the flywheel passes the forked prong at a given spot on the flywheel, the faster the wheel turns the faster the spark hence the engine runs quicker. Clen the rust off with a releasing agent (wd 40) hey presto!!!!!!!!!!!

Use steel wool to clean it, then be very diligent about putting olive oil on.