If its a standard 3.1 install you are correct, you should only need to copy over PC0 and it work.
PC0 should be in DEVS:DOSDrivers, CrossDOSFileSystem in L: and mfm.device needs to be in DEVS:. Make sure they are all in the right places as that would be the most obvious problem.
Also are you using your DF0 floppy drive btw?
If you are using drive DF1 then it would be PC1 you copy over instead.
