Here is my brother on his 1987 XR600R. Here's the problem. He needs a new back tire in the worst way. I can't find a decent back tire for the 17" rim though. We ride in a lot of mud and soft terrain. What would be the easiest way to upgrade to a 18" rim? Will a later model bolt on or would he be better off buying a new rim and spokes? Appreciate the help guys, hope you get a laugh out of the picture.