1973 cr250 rebuild.

So I have a almost complete 1973 CR250 (missing the gas tank). I have never done a rebuild, I feel pretty comfortable most everything except the motor. Gotta be honest, scared to even think of a complete rebuild top and bottom but I just know it's gonna need it. It last ran in 1986. Don't think it's ever ran since. It has been inside the whole time either garage or shop. Any and all help hints tips trick links would be greatly missed appreciated! I have already torn it down and and have a lot of things at powder coater. I took the engine and set it in the corner.