If the head has no issues and the gasket is good then its  all good, although the combined head and barrel is a bit of a lump to lower onto the pistons. The 3/8th whit case  threads can be replaced with a 3/8-16 UNC recoil kit part no 33060  which allows to use of the std whit studs. It was the cheapest way when i did mine ( many years ago).

Just get a 3/8"whitworth helicoil or recoil kit. That way the insert will match the stud thread. 

Hi Daniel,

yes you can remove the head/barrel en-bloc and leave the head joint and pushrods etc undisturbed but tbh it is a very awkward thing to handle and refitting it might be dicey as Robert says.

I notice that you said the stud was broken, not pulled out, so you should get yourself a set of "easy-out" stud extractors, the Dormer ones are good, drill a hole down the middle of the stud, until you just break through, squirt some Eazit down and screw in (left hand) the biggest extractor which you can.

Get plenty of heat into the crankcase around the stud and apply some torque, don't go mad, be patient and keep at it, the stud will come, then all you need to do is fit a new one.

I forgot to say, this all applies if it has sheared off flush, if it is standing proud once you have the barrel flange out of the way you may well get it with Mole Grips, or even a small pair of Stillsons, (don't tell my Dad I said so!), all the above about heat still applying.

Good luck!

I have never had much luck with "easy out" taps , They are likely to break leaving a bigger job. If as Neil says you have a bit of stud to grip then a stud extractor can be purchased for use with a socket set  The other option is to centerpunch and drill out ,starting with a small bit to allow you to get centered,you may finish up with a bit from a recoil set ,but it is possible to drill out leaving just a coil of steel from the stud to be peeled out. 

If drilling our broken studs, always use left hand drill bits. As often as not, the broken stud will unscrew itself as you drill. Also easy out extractors just expand the bit of stud you are trying to extract making it more difficult. Or they just break. A better bet is something like these ones

Tengtools Tmse05S