Great questions, sorry I cant find my XB literature but will try to answer based on memory alone so no warranty. It's been a longgggggggggg time since I was a used tractor and dealer and slept since I worked on them lol so don't anyone have a calf now if this isn't 100% perfect .............
1. How do you access the resistor between the power terminal and the coil?
Its on the back side of that rectangular phenolic brown looking block where the input terminal is located HOWEVER you can test its continuity WITHOUT having to physically get right to it (see below)
2. What ohm reading should it have?
No specs here but a pure GUESS its low ohms, maybe 0.5 to 1+ ohm or so subject to temperature THE MAIN THING IS IT CANT
BE AN OPEN CIRCUIT (see below)
3. What ohm reading should the coil have?
Again no specs, but a pure GUESS is the LV Primary is very low ohms, maybe 0.5 or so to a few ohms while the HV Secondary is several thousand ohms. Again cant be open !!!
4. What is the purpose of the fly weights and how do you determine if they are worn?
It for centrifugal spark timing advance, I cant describe how worn or not.
5. Is there an internal wiring diagram for the XB? I have done a little looking online but with no luck so far.
a) Here's the wiring. Its NOT rocket science. External power stud gets voltage from ign when ON,,,,,,,,,,current then passes through the internal ballast resistor,,,,,,,,,,,,then through coils LV Primary,,,,,,,,,,then through points when closed,,,,,,,,,,to ground.
b) You can easily test to insure theres continuity from input terminal to coil (via ballast which is maybe 0.5 to an ohm or so) UNLESS THE BALLAST IS BAD OPEN !!!!!!!!!!! Then theres maybe another 0.5 to a few ohms to and through the coils LV Primary,,,,,,,,,,,,,then its to the points terminal,,,,,,,,,,,then to ground when closed. BASICALLY unhook the wire to the stud and put an ohm meter there and the other meter lead to the points terminal and with points OPEN you should read the total of ballast resistor plus the coil, maybe a few ohms total BUT IF ITS AN OPEN CIRCUIT EITHER THE COIL OR BALLAST IS BAD/OPEN. Id try it with condenser removed just in case the condenser was bad/shorted. WIRING NOTE Condensor is in parallel with the points !!!!!!!!!!!!
NOTE 1) Even if a coils HV primary has continuity under a low voltage check and isn't open it can still be bad and HV breakdown can occur. You need a real HV coil tester to really test it even though an ohm meter can show its bad if its total open. A low energy low voltage ohm meter can show if its BAD/OPEN but cant really prove its good and wont still break down under temperature and high voltage
NOTE 2) You need a real capacitor tester to check a condenser, but still an ohm meter can show if its definitely BAD. To DC once its charged up it should basically read an OPEN circuit, if it shows continuity to DC (once charged) its shorted and BAD. Its in parallel with points
NOTE 3) As far as above, when doing the ohm meter checks I described UNHOOK THE CONDENSOR in case its bad and a short circuit. A coil can still fire with no condenser at all IE removed but a weaker spark and points will burn prematurely.
This isn't the exact detail and numbers you were lookin for but knowing how its wired (terminal to resistor to coil to points to ground and what and where condenser is ) might get you by. Post back questions, but I'm more into electrical then the mechanical works of that XB.
John T LOngggggggggggg retired Electrical Engineer and rusty so no warranty but I try me best to help