For 500cc, the general opinion of the best option, is to put a RG500 in a RGV250 or Aprilia RS250 chassis.

For best horsepower pr kg, I think a roadgoing Honda RS250 (anyone for sale ?) or NSR with RS engine would come close to the 500cc.

But it can't compete with the sound of a V4 2-stroke
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Poul
"If Life Gets Boring, Risk It!"
MC21SE, KISS box
MC21SP, HRC box, racing loom