this thread was wierd. i saw a reply from the op that said his plug was rusted in. that'd explain breaking it off. i don't see that reply any more, don't know where it went. in my experince, plenty of fasteners on boats that break off due to rust or corroision. not excluding plugs. specially with lack of maint. or water intrusion that's not approached right away. folks break off plugs in jet skis often due to dissimilar metals and electrolysis. again, especially when lack of maint. happens.
while i've never had the experience of breaking off a plug. an easy out would be my first try too. i've broken off my share of various fasteners. not spark plugs, but screw threads is screw thread imho, and an easy out is a good first start. my concern would be with metal shavings falling into the combustion chamber. easy outs don't always hook up to drive right away. sometimes causing metal shavings. ya could remove the head if that becomes a concern for metal shaving or any other spark plug debris that may fall into the combustion chamber. with the head removed that issue is n/a, i'm sure google could answer as to how to get the remaining plug removed. research easy outs on google too. there are different verities. all dodge shows the most common type imho. which is the type i've used. sometimes with success. sometimes not. also, some rust penetrate may also be in order. pb blaster or the likes. i've had pretty good success with pb blaster. fasteners i thought would break off for sure. only to be removed. imho, likely because i used a rust penetrate.
good luck.
drill with progressingly larger bits until you could collapse the sides of the threaded section towards the center and remove it.
imho, that'd be the last straw. do that with the head removed imho. imho, drilling is always the final last straw. there are no other options. if ya drill, ya drill right the first time or replacement parts will be in order.
