I am very experienced in tape recorder repair...but the first time I replaced a belt in a 909 about 12 years ago was a lesson in paying attention for the unexpected. It was a simple enough fix, however there are two ball bearings that each of the capstan flywheels ride on that will literally fall out of place upon disassembly and roll away...you know how when you drop a screw on a completely bare floor and you CANNOT find it! :) Just be careful...
Also, if you do things the unorthodox way and remove any components on the front panel or head block assembly, be very careful when re-assembling as the tolerances are tight and you can easily scratch some of the fascias if you are worried about things like that (I am a stickler for absolute mint condition stuff, which is a curse by the way...).
Best advice is that if you have no clue what you are doing, take it to someone who DOES know a little about the machine and/or tape decks in general.
Sincerely,
Jeff