I got this old bike with 2 stuck chainbolts, completely stripped the backside so the chainring bolt tool became useless. Ultimately I drilled it out, by slowly increasing the size of the drill bit (if you have a cheap drill like me, don't do it all in one sitting to let it cool down, mine started smoking). Go from the back so the clockwise rotation facilitates the unscrewing process. Go only half way to preserve the allen side. Every time you change bits, use an allen to check if you can unscrew it then. If you drill the whole way through, destroying the allen side, you just have to drill wider. If done correctly/carefully, there should be plenty of buffer space so that you don't harm your chainring or crank. Good luck! Oh yeah I hear putting some bee's wax on it might help it stick better?