Don't apply heat, the speedo has internal seals and it's likely you'll melt them.
I've always had decent success by using a centre punch, use it to make a indent near the edge of the screwhead, keep hitting the punch until the hole is a couple of mms deep, then hit the punch sideways in the hole, keep the angle quite steep or you'll just slide out of the hole, clearly drive it in the direction needed to unscrew the screw. The combination of the shock and the irresistible force driving it in the direction needed quite often works. Best done either in situ or in a vice. If removed a little oil down the threads from the reverse can help. Those screws are often not that tight, it's the damage to the screw head slots caused by rusting that makes them difficult to remove. Try a stainless screw in there so as to try and avoid it happening again and of course some nickel/copper grease on the threads.
If the punch hole ends up being useless after a few hits, just make another etc
