The built in standoffs in the 64 case are, as with the rest of the case, very brittle. I would not be surprised if they all broke off simply from the shipping journey. You can 3d print standoffs that you can glue into the case after you have removed remnants of the old standoffs. Its best to have a drill bit or toothpick or some piece of material that you can use to align the existing remnant hole with the new standoff. I have done this several times and it works great. I use JB PlastiWeld Epoxy.
Here is a
link on thingiverse to a case repair 3d print that includes the standoffs.
If you don't have a 3d printer I can easily print a set of them for you and mail them to you in a first class envelope. You can also buy kits at Retro Rewind and eBay. The kits will usually have the alignment tool and other case pieces that break.