The last couple of projects in the garage involved having to remove very corroded exhaust manifold bolts within a tight space. In both situations I needed to remove exhaust bolts which had badly corroded allen heads, primarily due to zinc plating failing and water being trapped (the bikes get used in all weathers).
Over the years I have tried many methods to remove bolts from saturating in penetrating fluid, mole grips, heat gun, gator grip and so on. The method that I find works the best for me at the moment is using an old socket and hammering over the corroded bolt head, and gently rotating bolt until loose.
I good coating of penetrating oil can also help things along if you leave it overnight before making your attempt. My video shows the process I’ve used on both of my first generation Suzuki GSXRs over the past few months. I hope you find it helpful!