Id agree with Andy on this. The NSR and Honda RS frames are too narrow and tight for an RG500 engine. The NC30/35 was the first bike I thought of with a SS Swingarm that would be a good option.

Ive seen an NC30 frame used to work with a 650 twin engine and have good room to fit it in.

if you really want to go all out, I am sure Harris would have something that could incorporate a SS swing arm no problem and be perfect for the RG500 engine.