Normally you would the optical reading would straddle both sides of the glass scale, one side being the TX LED's the other the P.C. receivers.
The problem with working on these is to get the alignment exactly right, the two sine/cos signals have to be exactly at 90°.
I think the best you could do is clean it thoroughly and check as many identified components as possible, replacing the photo det parts is going to be hit and miss as the components do not usually have any number on them.
Also you will need a double beam scope to do any kind of alignment.
If you have a scope, you could locate the photocel side and with the head mounted temporarily check the outputs for a sine wave when move slowly along the scale.
I didn't say that, just that apart from any fairly obvious problems, without the proper equipment and the ability to identify particular parts, it can often be a lost cause.
Most look for spare scales on ebay etc.