I would use a cobalt drill bit - they're extremely hard. Cutting oil is a must.
I have looked at a couple of different mounts, and the Russian military mounts offer secure attachment, but still let you tear the rifle down without having to remove the mount (not true for some other types). I believe some aftermarket stocks may require removal of material (plastic) to allow the mount to clear the stock.
Having scoped an SKS in the past, I have to say it turned out to be a fairly painless process, but also fairly pointless. The SKS is not a precision instrument, and you will generally not get nice, tight, groups at 100 yards when using a scope. Individual rifles may do better, so you may get lucky.
Still, the SKS is a fun gun to shoot, and I recommend putting one in your collection, scoped or otherwise.