Have had this little 138 at my shop for about a month. Walked off the lowboy and walked to where it’s at now. Never had an issue with starting. Fast forward until now and it’s not doing anything. I replaced batteries thinking sitting here could have killed them nope. So I took the ignition switch out and with a power probe on the acc side of the switch I can get my screen to come on but that it. Go back try key nothing no screen no battery relay click I got nothing and no schematics to trace wiring. So my initial thought is the ignition switch is there anything between the switch and the solenoid (besides the 2 fuses by the cutoff switch I’ve checked those) that could cause this no start issue.
Have you figured out the wiring yet. If not I can give you some tests to rule out buying some unnecessary parts. I don't have a schematic but some tests are standard. Do you have, or have access to a quality multimeter. Simon C