Either use a Propane torch to heat the screws and expand them a little, or use an impact driver to shock the screws loose. If you're working through the holes in a fiber timing gear, be careful not to damage the gear with the torch. If you're planning to replace the timing gear, just break it off with a big pry bar to give yourself more working room.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!