There are no real modern replacments for the RZ350 original shock since Fox stopped making them, but you can adapt a modern shock to fit. Once again this idea isn't my own idea, but I followed other's information on the internet such as this page, and found that there were some missing steps, so I figured I'd do it myself and fill in the blanks here. The yellow shock above is one from a 1999-2002 Yamaha YZF R6 and has adjusters for preload, compression rebound, and dampening. The original RZ350 shock is shown below and can only be adjusted for pre-load.
There more modern R6 shock should allow better control of the suspension set up, and has got to be better than the 25 year old original. The two main areas that need to be worked on to fit this shock are the top and bottom bushings, and the top frame connection. Looking at the tops of the shocks there are differences in the width of the mount and the size of the bolt hole through the mount. The R6 shock is shown on the left.
So the trick is how to get the R6 shock to have the same moutning dimensions as the old RZ350. Turns out shocks typically have bushings or even needle roller bearings at each end. Because of this you can remove the R6 bushings and using some new bronze ones, to make the original mounting bolts work on the R6 shock. You can see the shock above with the original bushings removed