First see if the piston is up and covering the ports. If it is, you can take a spark plug, break the porcelain out of it and thread it for a grease zerk. Of course I hope you have tried to use pentrating oils first to attempt to get some slipperiness to it. Any how, just use the grease zerk as a hydraulic pump. No sense trying to do it in one day, after all the bike has been like this since '71. But every couple of days give the grease gun another pump or two. Once you are down to the ports, then you "may" be able to remove the cylinder studs. This will permit the cylinder to lift off of the block. If that happens, you can now place some bolts with nuts around the base of the cylinder and the block and expand them like a jack. Work slowly all the way around. Use at least 4 bolts and nuts so you don't distort anything. Us some brass stock or even some good clean thin steel stock between the cylinder and also the block, between the bolt head and also the nut, so you don't get these mating surfaces all scratched up. DO NOT TRY TO PRY THIS APART. You will only damage the parts worse and probably also break some fins. Is this a Sport engine with aluminum cylinder or the cast iron cylinder?
