If there is anything still covered by ITAR in a 50 year old engine, then it really reflects badly on humanities pace of innovation.
Fissile materials, ofc.
But the US government took the approach that "secret is better than banned" with the bomb, and just never stated any of the details (as they would have been required to, in order to prohibit them).
MAD trade secrets, I guess.
You actually can sell a tank with a working cannon (to a civilian in the US, foreign national or not) if you have at least a Type 09 license to deal in destructive devices and the buyer has a destructive device permit. To export it would be a lot harder (and getting the aforementioned permits is no joke either).
Types of FFL:
Type 01 – Firearm Dealer/Gunsmith
Type 02 – Pawnbroker
Type 03 – Collector
Type 06 – Manufacture of Ammunition
Type 07 – Manufacturer of Firearms
Type 08 – Importer of Firearms
Type 09 – Dealer of “Destructive Devices”
Type 10 – Manufacturer of “Destructive Devices”
Type 11 – Importer of “Destructive Devices”
You don't actually need either the FFL or the permit if you just decommission the main gun, which is how most tank and artillery collectors get theirs. DD permits are rarely issued for explosives without a good reason (like construction or mining) so even if you get the permit for the gun, you'll struggle to get one for the shells.