I've done some looking and found a conversion where they have a kit that contains a new barrel and trigger guard, and bolt face to convert it using a single stack AK magazine, which I believe is from a Romanian AK.
It's a 100% mod that isn't easily reverseable.
It also takes some gunsmithing and head space calibrating.
The insert isn't a permanent mod, and can be removed and you can use the .303 just like before, but using the insert you would be using it as a single shot unless you did some modification of the mag, and mag area, as mentioned above.
I'm wondering if this will work with the standard bolt face that is original with the .303 since the cartridge is a rimmed one, and the 7.62 X 39 isn't.
On the Isaphore (India Enfield rifle) the bolt is for the non-rimmed 7.62 X 51 which I can see it would work, although not great, but it would extract the shell.
Just wondering if this is a doable venture and if anyone has done this with the British .303.
BTW the .303 bore is .311, as is the 7.62 X 39, so that is in it's favor.