You need the 89-90 RS250 carb manifolds.
Tyga rearsets are good. As are their pipes.

38mm+ are what the 250GP's are running. Even the CR125 MX bike runs a 39...

It'll pick up power at the top.
Oddly, on the Suzonkey RGV forum, a guy who put TZ carbs on found it gave better power at the bottom and better midrange than the stock 34mm, but only a little more topend.
I put Keihin 38mm carbs on my aprilia RS250. HEEeeps better topend.
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The Chopster