Well, you aren't going to convert to 535 for free, but you can always take your barrel to a machine shop and have it honed to 87mm and get a larger piston. That'd be the cheapest way.
If you're looking for cheap or free upgrades, you should probably take a different approach first. Better flowing air filter, drill out your exhaust, stuff like that.
If it was me, I'd put in upgraded cams since you're replacing them anyways. I'm not too familiar with what's available for your model though.