From the picture provided, the arbor appears to be in good shape (structurally, unless you're redoing it for aesthetic design reasons), but if it needs repair for stability, that is good to address before the weight of the vines cause more damage.

Grapes do not transplant easily or well, so a new structure would have to sit where the old structure is (in relation to the vine main stems) in order to be a suitable support for them and minimize root damage when the new posts are anchored into the ground. Since grapes fruit on second-year wood (stems that are in their second year of growth), it should not take too long to start harvesting fruits again if you need to significantly cut back the vine growth resting on the arbor in order to rebuild the structure. You can learn more about grape cultivation in our Growing Grapes in a Home Garden page.

Many factors can result in grape vines struggling, including being too shaded by a taller tree or building wall (resulting in both insufficient light and poorer airflow that's needed to keep the leaves and fruit clusters dry, which helps them resist infection), having stems that became too crowded (resulting in the same shading and air circulation problems), or a need for fertilizer. Soil nutrient levels can be measured by a laboratory soil test.

If wild grapes grow nearby, they may be harboring a common disease like Black Rot that can spot leaves and ruin a harvest, and grapes are also very sensitive to drift from certain herbicides (if any were used on a lawn or nearby planting bed, for example). Preventative fungicide sprays are the only recourse if Black Rot has been a problem in the past. A couple other common pest and disease issues are noted in the page linked above.

Miri