is that the locking bolts I can see at the back of the head casting?
Yes.

Re: long drill and tap - quick n dirty fix

In your lathe with the appropriate tapping drill (6.8mm?), drill into a length of 3/8" bar , grind a small flat on the drill, insert it into the bar and dot-punch into the flat - it should hold well enough to drill into the quill

Similarly, drill a hole just small than the square end of your M8 tap and hammer the tap into it .