A gun is only limited to what ever belts you can legally possess with it. I do not believe there is a stated limit if you use using original pre-1945 manufactured belts or links.
This gun uses M13 links out of the box which are post 1945 and replaced the pre-1945 M1 links - as originally used by the Browning M1919.
I do know some Bowning M1919's were converted from using M1 links to use M13. If it were me spending all that money, I would simply pay a bit extra for the M240 to be converted to run on original pre-1945 M1 links or another type of pre-1945 belt/link rather than M13.
I wonder if there are any types of pre-45 belts or links would work with the M240 out of the box?
As far as export, that's up to interpretation of ITAR rules by the State Department at the time but definitely possible. Since other belt fed semi's not convertable to FA have received (BATF semi only configuration approval letters) and State Department export approval, it is not un reasonable to expect this might receive similar treatment