Yeah, measure the diameter of the base and you'll probably find a blank one somewhere. But you might also find that the screws in the base (actually bolts), are in a fairly standard pattern and if so, most bases are available pre drilled for one of those patterns.
If you drill your own, you might want to very carefully center the router on the base, then use something to punch where you'll drill. Drill slightly oversized holes, then using a larger drill, slightly bigger than the screws you salvaged from the old base, and drill a second hole about half way through right over the smaller one. or reverse the two if you wish. That will allow you a little wiggle room for getting the base in place. Here's a drawing, side view, of what that would look like.
You will also want to use a centering pin to make sure the base is centered BEFORE marking the hole locations. The picture is of a centering pin in the router and you can also see how many pre drilled holes are there, which is why you man not have to drill your own. If you don't center the base,then you may have a problem accurately cutting a groove or dado because if you are using a straight edge and the router rotates a bit, the groove will change locations.
