Can you sandwich the two router plates together? You will lose the thickness of the manga lock plate in router height adjustability though.
You may also have to trim the width and length of the pictured plate so it will fit in hrough the hole in the router table. However that would prevent you from ever using it as originally intended.
Alternatively...
If you drill the blind tapped holes all the way through you could then use an appropriate sized bolt, with a ground point on the end to indicate centers on the new plate.
Is there a dimple in the center of the ¾" and 1-¼" holes you could use as center alignment holes by drilling them out?
Probably the 3 very critical dimensions are the 3 recesses because they probably align the rods the router travels on and the height adjusting rod.
How thick is the magna lock plate and how deep are those 3 recesses?
How critical is that big center hole? Will it just be a clearance hole or does it need to also be precise with the tabs and recess lip on the bottom?
Can you post a picture of the router lift face that mounts up to the plate? I'm interested in seeing how the rods specifically interface to the bottom of the plate.
I am trying to preserve the original plate just in case my efforts in machining a new one don't work out. That pretty much prevents the stacking approach and drilling through the blind holes.
The center hole is where the inserts mount, it is already there on the replacement plate. All of the other holes that need to be made need to be indexed around the center hole so that the router will be centered under that hole when it is mounted in the lift.
Here is the original plate:
This is the lift sitting where it mounts to the bottom of the plate:
Just a stupid question here.....you mention magnetic inserts in the OP and we're talking about a aluminum plate ?? I assume there's more to it than I'm understanding..
FWIW- If your not familiar with doing this kind of work I'd suggest finding someone with a Bridgeport. With just your basic shop tools you're going to be hard pressed to come out with the precision this likely requires.
The plate is aluminum. The top of the plate has 4 recesses machined around the center hole where magnets are screwed in with small allen screws. The inserts for the plate are steel and they have holes in the tabs that sit over the magnets so that the insert can be adjusted flush with the plate while the insert is in place.
So, I spent some time working on this today and I think I am making some progress. I was able to make a pretty accurate template from 1/4" hardboard. Only took two attempts. I've got the large hole locations laid out on the bottom of the Incra plate. I will lose one threaded hole that was there for a starting pin. I can drill and tap another one if needed down the road.
I found an old aluminum plate out in the shop and was able to experiment with some carbide forstner bits. They cut the aluminum just fine. Once I get the large holes drilled to the proper depth, I can put the lift on/in the plate and use the holes in the lifts mounting pads to locate the rest of the mounting hole locations.
As far as tapping the blind holes, I'll use a standard tap to get the threads as deep as they will go. I can then use a hardened screw to make the last few threads in the bottom. If that doesn't work, I'll just drill the holes through and counter sink the exit holes in the top a bit.
Hope this all works, if not, I will only have ruined one router plate and wasted a bunch of time.