Having been a diesel mech for 35 years and diddeled with many old bikes i have yet to see a trans rusted up.
Granted it could happen if it had been submerged... seen that happen to truck rear ends & transmission from submersion when hot and then it draws the water in.
Gear oil is formulated to prevent corrosion because quite often the oil just gets warm, never hot enough to drive out/off condensation.
The Tonti (later bikes) trans take some special tools for disassembly.
The rear end is easy to open up. Just be sure to count & "mike" the shim gaskets.
Do drop the sump as noted earlyer.