Dealing with frozen bolts is one of the hazards of working on these Honda's. Broken bolts will need to be drilled and tapped. I usually put the cover back on and use it to accurately locate and center punch each bolt I need to do. You can add tape around the punch making it just a hair smaller than the hole in the cover to make this operation easier and as accurate as possible. Goes without saying you need to be careful drilling those out. Drilling too far/deep will open the bolt hole into the crankcase....
Breaking one of the bottom ones is cause to have to do a lot of work. You'll need to pull the engine block pretty much to get access to those.
Replacing the block is an option as well. They aren't too expensive. You just need patience and about a hundred dollars worth of gaskets. The engine though, will be sturdy as new when done. Should last a long time. Rarely do you ever wear one of these out. It's always crap like this that you need to deal with. Engines run in salt water are really tough to bring back, but from the looks of yours, you don't have that issue to deal with.
I made a hobby out of taking a motor like you have and bringing it back to life, doing whatever is necessary to bring them back to near perfect running condition. Takes patience and a willingness to get your hands dirty, but it's rarely cost prohibitive as compared to what a running used one is worth.
