Wood Rot on joist and bearers

I have a timber deck about 14 years old and is in very bad condition. I started looking in replacing the decking but realized that there is wood root on the surface of the joists.
I also have large cracks from the nails. I have used a belt sander to sand of the top surface of the joist and the timber underneath looks still solid.
My questions is, what would be the best process in treating the timber for the rot and what would be the best product or procedure to preserve the timber?
What would be the best product to fill the cracks and gaps in the timber?
I also have some poles affected by water sitting on the surface that show some rot. Is it safe to seal the whole stump with a bitumen type sealant from further rot and what product best to use?
Replacing the joist would be way to expensive for me since the whole decking area is around a 100 square meters.
I appreciate any comment or advice.

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