As stated before anything can be restored to running condition in theory, but just because one 60 class has been restored doesn't necessarily mean the remaining surviving ones, or 5711 can all somehow can be magically restored to operate.
The restoration of a locomotive, regardless of size, shape, type or class is very much a case by case basis.
Just because two locos were built by the same manufacturer at roughly the same time does not mean that they will both need exactly the same parts and attention to bring them back to operation. The way an individual locomotive was maintained in service, and cared for after it's retirement will have a big say in how easy or difficult a restoration to service will be.
Some locos are withdrawn from service, and stored in running condition so if the railways ever needed to call upon their service again for whatever reason, they are fairly well "ready to go". These locos usually represent the best "value for money" in terms of a restoration.
Others are withdrawn due to failures of big components. Boilers reaching the end of their service life, mechanical parts of the locomotive becoming life expired etc. Some locos are that far gone that a full restoration would require almost a whole new locomotive to be built with the original road number stuck on the cab.
Let’s take 6040 as an example and break down what needs to be done to get that locomotive steaming down the main south under its own power.
First and foremost what condition is the locomotive in overall? I know for a start it's missing its air compressor off the side of the loco. (I do believe that a spare has been found, but not fitted). What sort of condition is that individual part in? Can spare parts just for the air compressor be sourced so it can be refurbished and refitted?
What sort of condition are the wheels and running gear in? 3642 recently had new tyres fitted at a cost of $70,000. That’s on a 4-6-0 loco. How much would it cost to retyre a 4-8-4-4-8-4 loco? Could spare tyres even be obtained or would they have to be custom made? How much life have the axle bearings got in them? All the connecting rods, what sort of condition are they in? If one of them has started showing fatigue cracks, can a spare be obtained?
What sort of condition are the locomotive frames in? The coal bunker, the front water tank unit? Was the water tank left full of water at one point and has rust started to appear internally that needs to be taken care of to make it water tight? How much is that going to cost to get that all fabricated? Even if an engineering company was willing to do it at "mate’s rates" the cost in materials alone would be huge.
What sort of condition is the boiler in? How much work is that going to take to be able to hold main line steam pressure safely? Boilers, unlike fine wine don't get better with age. If the boiler on the loco is too far gone, is there any spares available, and what sort of work would it take to get the spare ones up to the task?
Where can the restoration work be done? Who’s got a loco service road that they'd be willing to loan out for a few years so such a restoration can take place?
Who can actually drive the locomotive on the main line? Just getting enough drivers accredited would be a battle in and of itself before a restoration were to commence, and even if you were to get drivers accredited, how reliable would those drivers be? If the were volunteers, they could cancel at a moments notice, and the train wouldn't be going very far in a hurry. If they were paid drivers, how much would that cost the organisation running the loco?
That’s just a basic overview on what would be required to get that locomotive to run again. All that doesn’t take into account pulling the loco apart, and finding the numerous other dodgy repairs and problems that always seem to pop up that add extra time and cost to any restoration.
In my honest opinion, people seem to get a bit carried away with what "should" be restored. The vast majority of railway enthusiasts would love to see 5711/ 6040/ 3820/ 3813/ 5910 etc running again. To restore a loco you need time, money, patience and expertise. Most rail enthusiasts would probably at best have 2 of those 4 required things. I personally might have the patience required for a restoration, but I don't have the time, expertise, and I sure as hell don't have the money.
