It is possible to modify a metric receiver to accept inch pattern magazines, but why would you? Metric magazines are so less expensive, it would be a waste of time/money.
The magazine that arrived with the rifle has a brazed-on hook at the top front? If so, it is an inch style.
If the hook on the top front of the magazine was punched directly out of the magazine body, it's metric style.
Yes, Imbel did, and still does, make runs of inch style receivers. Federal Arms was selling these for a while.
To re-enable the hold open device, you can remove and replace the assembly ($12-$20) or you can try to replace the pin that was ground down. It's retention pin is the same as the magazine release's pivot pin and can be removed with a screwdriver from the left side of the receiver. It's a pretty easy operation, only needing a little fiddling to replace the pin.