This is just a guess, as I'm not familiar with the inner workings of your deck but one of the belts, maybe the drive belt, might be riding higher on a different groove, as the result of being stretched and this causes the speed issue. I can tell you for certain that a recorder of this age needs to have all of its rubber replaced in order for it to function correctly.
Take a look at Rolf's photo above. Where does this belt sit on YOUR recorder? On the outer or inner groove? How about on the large flywheel on the right, which attaches to the capstan shaft? Again, where is the belt riding? How tight or loose are the belts?