From the perspective of the US it would be incredibly easy. The US is very lax about what you can import in terms of military hardware. Once something is actually inside the border it can be difficult if not to remove from the country. There have been cases where non-functional mockups have been held after military trade shows.

The problem you are likely to encounter are: where the country of origin will allow it to go and what countries will allow it to transit their borders. For older military hardware like this unless you declare you are planning to import into some war-torn region of Africa there is unlikely to be any problems exiting the country with it. You might have to pay someone a small bribe if you're in a corrupt country.

As far as transiting other countries, it is basically a nightmare. You can't just carry military hardware across other countries. The only reason the US can move its hardware through other countries is because of defense agreements, etc. It'd be far easier but financially impractical to just airlift the thing out. If you could get it to a port and cram it into a shipping container that'd be ideal, but then you have to find a carrier that will carry military hardware. I can't imagine many carriers would be happy about that fact. If you could find one, you'd basically wait 2-6 months and then pick up your stuff at a port somewhere in the US.

Yes, Jacques Littlefield did it all the time when he was building the tank collection, however there are constraints. And one of them, especially for combat vehicles is that they are 'demilitarized' which generally involves some damage that would render them unfit to use in a fight. As I recall one of their tanks had the barrel cut off and then re-welded back on with a slight offset. Any attempt to fire it with a shell would cause the barrel to explode in a backfire. However some black powder and wadding was OK which gave a satisfying 'boom!' and smoke.

I think you can own a howitzer in the USA if you want