I've hit a snag...the head bolts on my OS are rounded...I can only get one of the four bolts out...it's a 2.5mm Allen key. Any ideas on what I can do?

I was thinking that I can heat up the engine with a hairdryer and also file down a 3mm Allen until it is a snug fit?

Also, should I use oil of some sort on the head bolt threads (once I eventually get them out) when I reinstall the bolts? The one that came out felt like it was binding and was very tight.

Thanks