Step one is to take the side moulding off and lift up the roof. Then you can look at what is going on.
If you are as off your rocker as I am, you will then repair the damage you find. Considering the condition of what you showed us, I expect that all the plywood has to come off the roof. IF you find that the framing is salvageable, you can treat it all with clear penetrating epoxy sealant (CPES). That will both strengthen what is left and kill any mold or fungus that settled in. Then you re-skin the roof and re-adhere the aluminum.
You absolutely should understand what everyone is saying. DO NOT pay for that rig. It should be free, or it should go to the scrap yard. I would ONLY consider a repair if everything else in the rig was in good shape and the engine was totally healthy.
I foolishly paid for mine and put in hundreds of hours rebuilding the cabover. I cannot emphasize enough how big a project that old rig looks to be.
Cabover rebuild threadJose