Finding older murray parts

I have a 1981 murray mower 11hp 38 deck

The model # is 81-36670

Though it runs well i would like to change the belts,blades and do a tune up

But alas a manual is not to be found and though i found a parts diagram with part numbers.

They all come up " item not found"

Any one know where i could parts for this older mower?

I would greatly appreciate it