There was a company that used to make replacement caliper brackets for radial brakes, but I've heard they no longer do this.
From what I've read, the RC51 has excellent brakes for non-radial design, and the biggest improvement will come from proper pad material selection and a radial master cylinder and braided lines.
Choice of new front end? Ohlins R&T forks of course!
As a side note, I happened to glance at one of Honda's concept bikes and it's forks looked exactly like RC51 forks (compression adjuster knob on front) except with radial brakes. Amazing.