If you send me a PM with your email address, I can email you a jpeg scan of the SL HiSpeed 510 exploded view. You can use this to dismantle your winch. (It can be either a 1.3mb bitmap, or an 80kb jpeg.)
With the demise of SL, I am not sure where to get spares for these winches.
I took mine apart about three years ago but I cannot rememeber how! It must have been fiarly simple - I think I just took off the cable drum and undid all the bolts around the face. Inside there is a system of bycycle chains and sprockets which provide the drive. I can rememeber that i got all my replacement parts from Halfords - £4.99 vs £22+ from whoever was offering the official kits! I do rememebr some tufnel (?) bearings which had to be replaced. I think I got these from ASAP. I needed new ones because one had seized up and I had to drill it out.
There have been several threads on these winches previously - perhaps a search may bring up the answer.