Santana overdrive

I recently came across a santana overdrive, seems to have quite a bit of
wear on the input splines but the box itself is good...
From what I have heard, parts for these are about as common as rocking
horse turd :-(
Anyone know where I can get a manual / parts for these?
How critical is wear in the splines?
Any info on these units much appreciated.

Cheers.

Steve.