Sounds like you've got Coda/Cannondale fittings:
The rotors are still available.
Otherwise, some 6 bolt ISO adapters could be made.
Sounds like there's a reasonable demand, from this
thread on MTBR last year.
The lockrings aren't strictly necessary...American Classic made their hubs without for several years then went to fully machined in ones.
You'd need to brake backwards as strongly as forwards for it to unscrew.
Fixed sprockets need them as the braking force is always trying to unscrew the sprocket.
All the best,
Pause, take a breath and ask yourself: "What would Rastamouse