As Ashish pointed out, the easiest way to regenerate OSCCAL is by using a Pickit2.
If not that, write a simple program to serially load values from 3400 to 34ff into the OSCCAL register, and monitor it (the value you've just written) on a serial terminal emulator (Hyperlink/Coolterm..)
You'll start getting garbage, until suddenly you start getting legible numbers. Make a note of the 34?? starting value, and the end value; the mean should be your OSCCAL value, which you can manually burn at the right address.
Crude, but it works. I've resurrected many 12F6xxs using this technique before the Pickit2 was made available.