Several helpers have pointed you to the problem.
1. Does not matter what electrical is working or not working because the starter motor gets direct full battery and starter is semi mechanically operated.
2. The issue you state is: "starter won't operate/no crank", (as opposed to a "no fire" issue).... so knowing how it all works tells you that it must be the battery, the starter, the button switch mounted on starter or cables. The only wiring involved is one positive battery cable and one negative battery cable. So now you know one of those items is bad.
3. Don't assume battery is good under a heavy load of a starter.
4. Don't assume cables and their clamps are good/tight/clean.
5. Don't assume starter is OK.
6. Don't assume button switch is OK.
7. No fuses or breakers involved in this.
Quick test is to jump the battery or use a temp battery from another vehicle. So try to eliminate the battery as the cause by using a known good battery....or have your battery tested under load by a trusted FLAPS or Shop.
The starter motor can draw 300 amps. This will break down a bad battery but then battery will be OK for small amp loads (bulbs/gauges). Also this amp load will break down loose, corroded, weak, broken, old cables, cable clamps and cable connections. Be sure the starter button mounted on top of the starter is still being pushed by the linkage. Report back.
Your battery needs 12.6V or near, at rest. It is the no. 1 suspect in this case.
Are we assuming that the necessary modifications have been made for 12V conversion? Spell that out for us. externally resisted Coil, starter button with side terminal, ballast, 6V starter..........What have you or anyone done since it last ran?
