Get a barrel extension. Torque it on until you've got it lined up with the existing index pin hole. Check headspace. If headspace falls within safe spec (for a used barrel like this, it can close on a NO GO, as long as it won't close on a FIELD), then you're good to go. If it doesn't, there's basically nothing you can do about it unless you can weld up the old gas port and drill a new one.
If the headspace isn't within safe limits, and you're feeling adventurous, you can do the following:
1) relieve the barrel extension threads on the barrel so you can torque the barrel extension on a teensy bit more, if necessary
2) continue tightening the barrel extension a bit at a time, checking headspace as you go, until you get the headspace within safe limits.
3) weld up the old gas port and sight base pin grooves (you can weld up the old index pin hole if you like, but I wouldn't bother)
4) time and drill a new gas port and index pin hole.
5) time and install the sight base, and drill new grooves for the sight base retaining pins.
The need for a lathe is to smoothly relieve the extension threads on the barrel, if necessary, and to clean up the excess weld from filling in the old gas port and sight base grooves.
When you're done, if there was a headspace issue, you've got a really nasty barrel throat/leade, but it should work all right. If you were lucky and headspace checked out using the existing index hole, gas port and sight grooves, it's just like any other used barrel, I'd reckon.