You got it as a project right? So don't give up on it straight away! Add up everything the bike needs and the tools you'll need to fix it and see if it's worth doing. If it is I'd take the head off and bring it to a machine shop and get them to get the studs out. That's the easiest and best way to do it imo. I'd be surprised if it was more than £50 to get the studs removed and new studs put in. If you get it done properly you know you won't have anymore problems with them in the future. Plus you can do your valve clearance and check the condition of the valves, cam chain and cylinders while you're at it. Add in the price of a new head gasket to the studs and you're sorted. Or you could have a go at getting them out while the engine's still in place. It's a bit more awkward but possible. I think it was deadeye on here who put up a link to an extractor and impact that worked for him. Another option is, I've seen someone on here use a small threaded sleeve that they screwed over the end of the snapped stud to give some more threads, then screw a new stud into those threads. That's a bit of a bodge but it seemed to work for them