Are you talking about the rubber boot that goes over the pushrod and slips over the back of the cylinder, or a gasket between the firewall and the m/c casting where it bolts to the firewall?
If it's the latter, the easiest solution is to make your own gasket. Your local parts store will carry gasket material. Cut the center holes to fit it over the bores, then use a ball peen hammer to lightly tap all around the perimeter of the casting and it will cut a perfectly formed gasket. This method also works for cutting the mounting holes in the gasket.
