This all depends upon the question of WHY.
If you are set on drilling the hole yourself, good luck.
This operation is hard enough to do correctly without scratching the half-silver coating and getting to size correctly. I've done it right twice and has more failures due to the glass age, imperfections, tiny weaknesses, etc. I've never drilled a hole through a half-silvered mirror though, so that's awesome if you can get that done.
Aside from that, I'd also recommend just buying the correct part. If you take the cost of all your salvaged RFs, the tool cost and the cost of your time and effort, you are going to have a thousand dollar mirror by the time you're done successfully.
Unless the whole point of this exercise is to do every step yourself, then it's just down to being a craftsman who happens to be working on a rangefinder part. The latter course of action is admirable and you'll have a piece you can really be proud of when you're done but in the former case, this is not the way to save money or time.
Like building one's own large reflector telescope from hand grinding raw glass blanks, silvering the mirrors, precisely setting up the chassis and getting everything dialed in. Ultimately satisfying when you're done but if you just want to look at the stars, a good used Newtonian with a couple eyepieces cost pennies in comparison.