I presume it's a low-pressure boiler, which are usually set around 5-7 PSI.
If it's higher pressure, it's gotta be done right, not patched.
Is it in a place where you could install a repair plug ?
Or is it small enough to drill and tap the hole and install a pipe plug ?
From: Jim Franklin
Subject: [Shop-talk] Welding cast iron boiler?
My steam boiler has a 1/4" hole at the top. Predictably, all the
tradespeople say it can't be welded, must be replaced. Also predictably, my
new landlord doesn't want to replace this, so she's looking for the lowest
estimate, which will get her a crappy install, and problems very soon. Can
this be welded to buy her some time, ideally to the end of April or so?