I suspect a welder would just want to be given the crankcase half to work on so that would mean an engine strip. I'd be surprised if you could find anyone willing to work on an upended complete engine. Even if you did get the area built up with weld it would then need to be milled back level so the sealing surface was flat. That's not going to be cheap in the UK and may well exceed the sentimental value of the bike.

Personally I'd go down the JB Weld route. The bolt's only purpose is to allow you to drain the oil so I'd firstly drill another smaller hole right through the bolt and tap a thread into it so another smaller bolt + fibre washer could be fitted / removed for oil draining.

I'd then clean up the entire area with solvent and epoxy the original bolt in place - including the thread area. If you have the pieces that broke away I'd epoxy those back in place as "spacers". Getting the whole area clean and roughened up to give the epoxy something to "bite" into would be crucial though. I'd even consider drilling some small holes into both bolt and crankcase mating faces to give the epoxy more interaction area. If you don't get it clean the epoxy won't do much more than a superficial stick.

There are various grades of epoxy and this (IMHO) is not one for the five mins or one hour stuff. You need the "takes a day to two but sets like concrete" stuff. Just don't get any of it dripping inside the engine!