I've done this before on ET engines. It's more than just drilling the hole out. It must be machined. It can be done without dissassembling the whole engine. You need to take everything off and get it down to a long block (crank case, crank, cylinders, heads) I machine shop will then have enough room to clamp it in a mill and flycut the surface where the starter goes. Once the material covering the hole is gone your starter should slide right in. The trick is to take off the correct amount so the starter drive works with the ring gear properly. If you have an E-start block laying around just duplicate the distance from the face where the starter goes in to the recoil end of the block. If not you will have to install the ring gear into your engine. Get measurements from it to the recoil end, then measure the starter gear to mount base measurement. Allow for clearance and add them up.
It isn't a hard job but not a ten minute quicky.
opsled