I think that, with the single user mode, but maybe another solution.
Make an new installation (VMWare, or hard), so root and password.
Mount your HDD (root account lost) from your new install. copy 2 files:
Modify only the root's line on every file with the good line from the new install.
Yes only on single user.
I don't know how .... on the old unix, on starting boot kernel, make a CTRL + d, and the kernel start on single user mode (root by default), and into this mode, you could make passwd command. Now, I don't know if is the same thing! :huh: