Are you sure that it's a 250RSA that can be converted or is it the 250Superdream Twin which was derived from the Honda CB400 Superdream ?. I would doubt that the 250RSA could be converted without major and uneconomic work being done. If it's the Superdream I expect that it could be done but most of the internals, crank/pistons/gearbox would need changing, better-off trying round the breakers for a complete 'lump'. Some people have successfully transplanted an XBR500 power unit into an RSA but the increase in performance is nowhere near as much as you would expect and again hardly worth the effort.
