I don't regret selling mine. Don't get me wrong I still loved the bike and wish I hadn't let it go for sentimental reasons, but as a racebike it wasn't logical. Parts were were getting harder to find and more expensive. Not to mention that it was outclassed in every category except for my personal preference towards it. If I were doing trackdays only I would have kept it just to putt around on as it would do 17's at Putnam all day long. After 5 years on it, I bought a new 600 and immediately went faster. If I were to go back to the twins, which I will eventually do at somepoint, I will be on an 848. Outside of Road Atlanta, don't need anymore than that.
this one looks good if you want to go the rotax route (1060 kit) http://www.apriliaforum.com/forums/showthread.php?t=210158