It looks like a case cover , so if it were me I would try an epoxy repair (get one of the epoxies that has metal in it) but you have to find the piece that broke off!
I got the piece. I drilled and tapped it deeper. Problem is the oils seems to work it's way around the bolt, and drips into the pan. I RTV'd the shit out of the end of the bolt that meets the cover, and it seems to be holding, but I need a more permanent fix other than replacing cases.
It needs the busted off part for the gasket to seal against. That is the piece that I would build up with epoxy, not for strength, but just for the gasket to sit against. Put a bolt in the hole (without the cover on) where the head sits flush with the case, build the epoxy part up and let the epoxy partially cure, and then take the bolt out, and there are your threads. Good luck
Short of welding on a slug (or building up with weld), drilling, tapping, filing, etc... you need cases.
Weld on the broken piece and clean up the matting surface with a flat file and clean up the threads. It isn't (really) structural and there is no oil pressure. Just make sure the welder is good with small stuff and not just the local Caterpillar stitch-an-go guy.
see if a good welder can weld an aluminum bolt in there. presto its now a stud after you cut the head off.
When my kid lost his chain at NOLA, it broke off two threaded lugs and put a small hole in the case. I took it to my motor builder and he took it to his welder. I think it was $100, but it may have been $200 and it worked great after that. Motor only had 1 or 2 races on it and still ran strong after the welding. I was worried it may cause too much expansion and create issues, but it didn't. Good Luck
I used something similar to this. http://www.alumiweld.com/h2uaw.html Works amazingly well and strong. Just screw a bolt into whats left of the threads. Use a washer at the head of the bolt to get a reasonably flat mold for the aluminum "soldering" on the casing surface. Just apply the aluminum around the bolt and the bolt will unscrew with the threads in place. I have seen a couple YouTube videos of people doing this exact repair but couldn't find them with a quick search. I don't know that I would use this in a highly stressed repair such as an engine mount but for this it would be fine. Harbor Freight and Ebay sell similar alu rods as well. Oh, and practice a few times if you decide to use this. It's a little different to work with than welding or brazing.
With the oil and case paint in that area Zinc brazing is going to be a pain in the ass. But it does work on a 300zx convert top!
Opinion from a race engine builder - Proper racebike repair - New cases or new engine. Next best repair - Fixture case and locate hole center, weld up damage, refixture, drill, tap, surface, install time sert (Requires disassembling cases). This is not going to to be a solid repair if you try to epoxy, patch or fake it, you will have leaks - eventually.
a piece of 6061 8mm rod. add threads get it welded in there. no worries of something coming loose, and there is plenty of flat surface for the gasket to seal.
I respect the heck out of Rick at RM. As a lower budget non-factory guy, I chose welding it and testing it before a big race, although replacing would have been the best solution if money were no object. I tested first, because when running AMA, the travel and hotel is such a huge expense, you don't want to throw that away on a silly "cheapass" decision of repairing the case. Since I had the backup motor I thought it was worth the risk. Fortunately for us it worked, where new cases would have been so much more expensive, as once you do that, you may as well rebuild the entire engine.
