Hi, Steven.

Considering that (1) the purpose for the alignment pin is likely to aid in speeding up factory assembly (after all, once the lug bolts are in, it's not going anywhere) and (2) that the pin was dropped later in the 124 model run, I think you are perfectly safe to remove it. I believe it's simply pressed in, but at that age it may prove rather difficult to remove. If you try to grind it, you must be very precise, since it overlaps the mating surface and you certainly don't want to damage the hub...nor do you want to leave even a hint of raised bump behind (or the rotor will not seat properly).

Personally, I'd try to remove it before ordering new rotors, and if that fails, then you have to decide whether you have the time, tools and talent to properly grind it off. If not, get OE rotors.

Of course, if you are a purist, leave the pin in place...but if you were a purist, you wouldn't be asking that question.

As to which rotors to buy, I would use the EPC to look up specific part numbers for your car by VIN. Then search for that part number on line, and it should have the pin hole. I suspect part of the issue you are running into is that places are stocking "one number" for convenience which fits later models, but omits the pin hole for older chassis (likely a lesser manufacturer doesn't know the older models have the pin and it's cheaper to make the rotor if you drill one less hole).

And if you're shopping on flea-bay, well, don't expect much.

Good luck.