How to rebuild honda HR173 motor no compression

Hi
Just took apart my mower motor it looks new just have to take valves out of it..
Only has 20 PSI compression, maybe head gasket will get new one and broke
a head stud will drill out and put a new one in....
How do you pull out the valves?
PS it is 15 years old and is it worth rebuilding?

Cheers Alan