Bisset

Im new to the forum so il start by saying HI. I have just got myself a DTR 125 to learn to ride on, I bought as a non-runner and have now got it running, however it will only run with the choke on! I thought this would be an easy fix with adjustments to the pilot screw. This is where my problem starts! the pilot screw has had the head ruined and I can only just get a bit of grip on it but the screw doesn't want to turn with the small amount of grip i can get. Does anyone have any ideas on how i can remove this screw without damaging the Carb? any help is very appreciated!