Change the stock exhaust for some Jolly Motos or other brand, the Jolly were 11,5kg lighter than the originals....
There is a fault in the specs for the RG500 on this site, it doesn´t have an inline four, it has a squarefour, ie two twins built together in a square. The cylinders are standing straight up. I only know of an old Ariel that is supposed to have this kind of engine. Cool. The carbs sits on the crankcase from the sides...