Head Gasket

If the rocker cover gasket is not leaking then leave it alone would be my advice. But if you are determined to do the job then from your description removing the rocker cover should reveal the remains of the shank of the snapped bolt. If there is enough of it to allow you could use a bolt removal tool or even a good quality mole grip. Preferably the actual make MOLE grip as the Chinese copies are crap. Or if you can weld then weld a nut on the shank and screw it out. If that fails then cut the shank flush with the head and centre punch the remaining shank and very carefully drill the centre. Use cobalt drills and start with say a 1.5mm and build up to just under the shanks diameter. If you have been accurate that will leave you with a hole that you should be able to retap without damaging the head. Hope this helps..

A more effective prep for a long trip would be to run a flushing oil through the system and then use a nice high quality oil in the system.

Suzuki 1300 engines love good quality oil changed regularly and will seemingly run for ever if loved.. The penalty for not doing this is the likelyhood of an oilway above number 4 cylinder (the one at the back) getting blocked and the camshaft seizing.