Pictures worth a thousands words - fix depends on extent of damage.
Your first goal is to stop the leak - fixing doesn't make sense until that's done. Are you absolutely sure where the leak is coming from - cab over leaks can come from places other than the front windows - running lights, side seems etc. If you have any doubt where the leaks is coming from consider having the rig pressure tested - cost about $100 and worth every penny.
My cabover leak damage was modest - damage was mostly cosmetic (wallpaper, veneer etc). I scrubbed everything with bleach etc then used heat and fans to dry everything. Painted all the wood with a water proof paint. Purchased thin luan (stuff they skin doors with) as replacement for the veneer, replaced wet insulation etc. I took my time and it wasn't very hard.
In the interim - park the rig so that the nose is slightly elevated and put a tarp over the cab over area.
