You cannot adjust the tensioner. It is automatic, and takes out the slack of a stretching/aging cam chain. Once the tensioner has fully extended, the only fix is to replace the chain and the part of the tensioner the chain rides on if it is worn.
A stock Honda XR600 will hit speeds of up to 90 miles per hour. There are ways, however, in which the bike can be modified to make it reach higher speeds.
I´m the owner of 1991 model and I have made a rebuilding, and I think that the rear shock can interchange for for all the models between 1989 and 2000