I have a 1984 Barth Regency with a front shock absorber missing. How do you replace this bushing with the upper shock mount still in place in the shock ???

Not sure what you are referring to. Could you post a pic?

I hate doing this but what you asked made no sense to me. First the shock is missing. Second what bushing. Third shock mount still in place in missing shock.

You need to rewrite this. At least for me. Maybe others understand.