Mast hole repair

I have a small, trimaran with mast sections used to connect the hulls. The previous owner drilled holes approximately ~7/16 inches in diameter, clear through the spar, in some high stress areas. Although I see no evidence of impending failure, thoughts of a weakened area is giving me nightmares. These mast profiles are no longer available. I am not too keen on the idea of internally sleeving this area as I have strong doubts about my ability to remove castings attached to the end of the spar without damaging it. Finding a suitable internal sleave would be another problem.

I understand that the heat generated during welding can weaken the spar. I am not overly concerned about the appearance of the repair / strengthening. An external patch would be fine. Can a strengthening patch be applied to seal the holes and reinforce the spar, or will the act of welding / brazing / soldering weaken the spar?

Any advice would be appreciated.