I think it is time to look for a replacement cylinder on eBay. They will be a lot less than $450, even with shipping. I think 86-88 will work. I was at a swap meet in Woodstock, IL on Sunday and I think I saw 2 or 3 of that vintage. I would have picked one up for you, if I had known.
What size piston do you have in there now? The largest you can go with the stock cylinder is 68mm. If you have stock (or smaller than 68mm) you could of course just bore it out (and replate if it is a plated cylinder...???) Also does the bike have a sleeve or is the cylinder plated?
you can go up to 72mm if you want to have it boared out. that will cause you a lot of pain as carb rejetting and other mods may then be necessary. your best bet is to find another cylinder. ebay is great