I’m not a pro engine builder but I’ve rebuilt and successfully raced about 6 Leopards and I don’t think the sort of interference you’re talking about is possible with correct parts and assembly. I’ve never measured it, but there is ample clearance from the top of the piston to the top of the cylinder, and the concave shape of the head should make contact impossible. Something is seriously out of spec and I’d be concerned about interference popping up again after the parts heat up and expand, as the available gaskets are not that different in thickness. You should never have to fashion your own gasket to create clearance on this motor.
I know you want to save $ but seizing the motor at 15,000 rpm will not be the way to do it. I’m a DIY guy all the way, but as Dirty Hairy said “A man’s got to know his limitations.” Sometimes you just gotta bite the bullet and pay the man. Heck, for all you know you’ll get it so the thing will rotate again and you’ll only be back to it not starting.