If you want to stay original, I would think it is rebuildable. If you bore it and freshen up the crank, it will be almost good as new. People like Egge sell pistons for these old things pretty reasonable. If you are worried about it not to hold up to your use, quit worrying. I am old enough to remember when these things were new and they would go 50k miles on straight mineral oil between ring jobs. And that was full throttle all the time. With the lubricants we have these days, they will last much longer