^ yea no problem. i would go with the rafters and diagonals if you have some sagging. if you need more in depth explanation on how to sister two joists let me know. the other thing i would just like to add is you dont need to go overkill on the rafter size…generally a 2x4 is a bit undersize, but for your needs i think thats perfect. if anything dont go bigger than 2x6. i would also probably only do it every other roof joist in your case as it doesnt really need much more considering its an old design that has held up for 50yrs and adding so much of the new tech is overkill. maybe even every third joist.