here is my take on this.
The RS newer or older is just like the NSR. It's a V-Twin 2 stroke. Has power valves, inline intake, water cooled, etc.
The rest of the differences don't matter and won't make a difference in your ability to work on the bike. it's all the same in theory which is what is important.
As for the older bike on eBay. I am sure that bike would be a kick in the pants fun. However I would ask myself what are my goals of a track day. Have fun and ride a 2-stroke? or to have fun, ride a 2-stroke and learn skills on a bike that is a little more up-to-date.
Suspension, Chassis, brakes, tires have come a long way since 1991, and you will find that with a newer RS. As you have already mentioned, the parts are also easier to come by if you by an NX5 or NXA model. All depends on what you want.
i will say the 91 is a good looking RS with exception of the girlie paint color
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Charles Gallant