You've got a few different things in there:
Just to clarify, if the engine 'won't crank at all', I take that to mean that the starter won't energize and engage; no rotation of the engine.
If that is the case, then start with the starter circuit. Check for battery voltage at the b+ terminal of the starter, while trying to start the engine.
Check for a b+ at the start wire on the starter, while trying to crank the engine.
If both those are good, then you need a starter.
If the main b+ goes away while trying to start, then you have a bad conection (most likely) or a bad cable.
If the b+ never gets to the start wire on the starter, then its things like the neutral-start switch, ignition switch or relay. (not 100% sure that there is a relay in there). If you have an alarm or command start, they could both be possible casues of no power to the start terminal...
If the engine is cranking, but won't catch and run, then I'd suggest checking for spark while cranking. If no spark, then go after crank senors etc...
If you have spark, then check for fuel. No fuel means fuel pump is most likely, or pump resistor.
Hope this helps...
Way too many mods...
Currently tuning an HTV-1900.