Tell me that at on the cam spacer at between 1 and 2 o'clock that is a scratch and not what looks like a cracked cam spacer. That will happen if the inside bevel is to toward the cam journal because of the cam snout radius, in the pic it looks right but not sure. If it is cracked I believe ''Classic Inlines'' has some.
No I would not try to tap that at this stage of build, the soft plugs seem to work ok if new and not over pounded so that it looses tension. It needs to be all the way in, past flush but not deformed. I have in the past have used Permatex sealer, the brown goo number, but I wonder if red lock tight might be better, seek others advice on this.
If you do have to take the cam retainer off, I would like to know if there is oil groves on the back side of the plate and if you can tell us if, by way of witness marks that it come mounted as showing from the factory?