The track that this door slides on is worn out beyond repair. I need to replace it and I would like to know if it is possible (from your experience) to totally replace the rollers and the track. I have looked online for such kits but they don't seem to be available. i can't see how I can replace the track only as it is impossible to find one online so I am thinking about buying the whole thing (rollers and track). My questions to you: are these pretty much standard ? Is this type of replacement possible, do you foresee any issues?
The only thing "standard" about sliding doors is that they have rollers and tracks. Beyond that there is no such thing as standard. If you start to look around for replacement rollers themselves you will find out that there are literally dozens of sizes and styles. Check out this web site to see what I mean. They show 114 types that they have available!!
So to replace rollers alone entails having the part you are replacing in hand and very carefully compare it to the size description and pictures at the place you hope to find the replacements.
As far as track replacement, if your track is not too far gone, sometimes it is possible to purchase a clip on stainless steel strip that fits over the worn track and provides a new true edge for the rollers to run on. Not all tracks are compatible with these strips though and that leaves you with a huge decision of whether you just replace the whole door assembly. That is likely the only real choice because for as many styles of rollers that exist there an equal number of models of aluminum extrusions that the door manufacturers have used for the track and lower threshold of the door.
There is the additional factor regarding the replacement itself. If you look closely at most sliding door track pieces you will note that they are assembled to the side parts of the door frame using screws up from the bottom side. In the case that you did somehow get lucky and found the exact replacement track piece you would have to completely remove the door frame unit from the rough opening to be able to install the new piece. At that point you are half way toward what it would take to install a new door assembly.
If your house is old there may be some good arguments to install a new door. There have been many advances in these doors over the years including more energy efficient glass pane stacks and vinyl frames.