To find out if the switch is what's causing fuses to blow try these troubleshooting steps....
Is the switch mounted to metal or Fiberglass? If the switch is mounted to metal AND the case of the switch is shorting internally to +12 then, yes the switch can be bad. If the switch is mounted to Fiberglas (insulator) then the switch is most likely still good.
If swtich is mounted to metal - Try un-mounting the switch and have it "dangle" (case not torching any metal). Turn switch, if fuse still blows then switch is NOT bad....Whatever load is down stream of the switch is what's drawing more current than the fuse is rated for. Disconnect the glow plugs if it has them to rule those out. I suspect the starter solenoid is corroded internally shorting out windings. Disconnect the energize wire from the solenoid, turn the key several times see it the fuse blows then.
Does it have a clutch safety switch? (the schematic shows one) The clutch safety switch or one of it's wires could be shorting out to ground.
How many amps is the fuse that's blowing? You said it's blowing two fuses right? I can read the schematic, if the fuel gauge fuse is blowing then perhaps the fuel gauge is somehow shorted, I think you said that wasn't working anyway right? Perhaps that's why? Disconnect and tape off any and all wires going to the fuel gauge, to eliminate that drawing any excessive current. Through a process of elimination you'll find and fix it.
Larry