I replaced my grille with the early metal mk1 grille type a while back. After re-doing the grille I set about trying to attach it as the mounting points are not the same. To start with, I just cable tied it in each corner of the grille to the nearest mk2 mounting points.
I came up with a solution quite soon after. My father had found some metal plates that were the perfect size for going from the holes in the mk2 to go and bridge the gap to the mk1 grille mounting points. After modifying the plates slightly and making sure the holes were in the right places, just secure it up!
The center holes are the same for the mk1/mk2 so that's easy enough
The front facing mk2 points are the hardest to fit I found. There is not much room to work with. For this design, we bent some metal to shape, where it screws into the mk2 mounting point and joining to the flat of the bottom of the mk1 grille. Again, drill holes and secure, though securing this one is a bit more fiddly given there is not much space. If you're taking the bumpers off, I imagine you can come up with a good solution for keeping it extra secure on the bottom there!
As you may see in the first picture, the mounts bend down slightly, I believe this is because of the front facing mk2 mounts. The mounts on the car get in the way and almost prop up the mk1 grille because the mk1 grille has extended sides. One solution would be to trim the sides of the grille I'd imagine, which would let you mount it in a position more of your own choosing rather than where it wants to sit. Not saying it doesn't work without modifying the grille though, here is the finished result!
I plan on repainting the grille one day so I'll end up making the mounts a bit prettier too, but there's a rough guide to it - hope that helps, but yes, long story short, the mk1 grille can fit if you make the mounts to fit!