leaks in a corregated metal roof at fasteners

There is a corregated metal roof on an old quonset style building, formerly used by 4-H, that is part of the North Hero Parks and Recreation responsibility. The sheeting itself is in great shape, but the problem is with some of the fasteners. The old nail type fasteners have lead washers and over time some of them have started to leak. which then has caused rot in the wooden framing loosening them further. What is your suggestion for fixing the roof. We have a constrained budget. I once estimated how many fasteners there were and there were hundreds. Thank you for your help. Stephen Ludwig