First choice would be a 5/16" or 8 mm square wrench, as best I recall.
If that doesn't work ( which it frequently doesn't) drill them out with
a 3/8" drill and then go to work with a small chisel. If they don't
come that way go up by 64ths till you break through into the threads
on the far end of the plug and you should be able to get them out fairly
easily because the compressive stress will then be relieved.
Along the way if you have a screw extractor, feel free to try it but
GO EASY!! They are real boogers to get out if you break one.

Replace with double taper allen pipe plugs and teflon tape and don't
tighten them too tight.

A large calibre rifle brush or a small calibre shotgun brush cleans the
gallery pretty well, and if you have lots of bore cleaner, it works pretty
well too. Otherwise kerosene, PD-680, varsol, mineral spirits, or if you
are a Brit, Paraffin. If you've been using good oil and changing regularly
there shouldn't be too much to clean.
--
George Downs, The "original" Walrus3, Bartlesville, Oklahoma