If it has ONLY a 1/4 inch collet, time to buy a new router. My fave is the Bosch 1617, which has both a half and quarter inch collet. In other words, you can drop down a size, but not step up to larger. Beside that, the Bosch has the ability to mount under the table and be adjusted through the top. Sorry for the bad news. Don't drill your plate because not all bolt patterns are the same.
One more thing, when mounting your present router, did you remove the plastic base plate before mounting it? That will give you a quarter inch or so more. Also what size is your opening in the plate. It should be large enough for your largest bit. You use an insert to fit the opening to the bit, which is pretty tricky to make work yourself. That's why most of us just bite the bullet and buy a pre-fitted plate. The pix is of the Woodpecker plate which has a twist lock insert. Kreg also makes plates with twist lock inserts (Blue plate pix), much cheaper that the woodpecker BTW.
The more I do, the less I accomplish.