Do not lose heart. I have had the thread go on my xt 600 in this particular place . As cynic stated , if it doesnt leak oil, you should be ok. And if there is any thread left ,even better. The two bolts in this picture are not the main ones holding the cylinder head and cylinder in place and if there is no leakage the bike will be ok to run. If you are going to split the engine (and the thread is broken) then buy a helicoil kit. I believe that these bolts are a 6mm thread and a 6mm helicoil kit is a wise investment. 6mm threads on japanese bikes are very common and fragile. Especially if a bolt has been over tightened. With older bikes there is a good chance that you will need to do more helicoils and a kit will pay for itself after a couple of repairs.
As for compression i would be happy with anything over 120psi/8 bar and wold probably be ok with over 100psi or 6-7 bar.
And yes they can be buggers to start. Your model i think does not come with a kick, Just the blanked of casing where the kick used to be (i think). I have an 1984 model with just the kick and decompression lever. Full choke, no throttle push it just after tdc (the point of highest compression) and she starts. Any whiff of throttle for the first 30 seconds and she will die. With your bike i would spin her over with the decompression lever pulled (again full choke, no throttle) and once the engine is spinning nicely let the decompression lever out and she should come to life.
Good luck. Your bike actually looks in quite good condition compared to mine.