No longer worth the time, hassle, or aggravation. First there is a ton of time finding all the parts and the cost to do so. Then there is the fact that there are guys setting up stock Banshee cylinders up to produce just as much power with just as good of a curve. Plus with the power valve set up on the RZ cylinders you can only do so much porting and if your wants/needs ever surpass that point it will be a banshee cylinder you replace it with. The RZ guys are on a revival and it is because they are now starting to adopt banshee tech to their archaic RZ motors. They were one in the same, but it was the banshee that made the aftermarket really keep stepping it up.
So in turn, if it were me. I wouldn't bother with the RZ set up. Once upon a time it was gold. But now there are so many different builders doing excellent 4mm stock cyls, aftermarket cylinders for drag, dunes, trails. Almost any of those set ups will far surpass what you can do with the RZ stuff.