I have several pairs of these pedals, I'll not go into a whole bunch of detail but basically you should grind off the rivet heads carefully, and then drill and tap holes in the pedal shell/body (for screws): therefore fixing the cages with screws as opposed to rivets.
Seems an awful lot of bother if this is something you cannot do yourself - why not just buy a decent set of pedals, keep the better of the two (you already have) for second best and flog the other pair!
Just trying to help.
Co-founder of a V-CC section, member South Eastern Road Club and owner of - 2005 Bianchi Reparto Corse (modern), 1985 Raleigh SBDU Pro Super, 1984 Raleigh LU Corsa, 1980 Allin Stan Butler Special, 1978 Ron Cooper, 2 x very early Roberts (late 60s/early 70s), 1975 Bill Philbrook tourer, 1966 Raleigh Superbe Roadster, 1964 Allin Stan Butler Special Belgique, 1951 Hobbs of Barbican S/C, 1950 Hobbs of Barbican S/C, 1947 Hetchins Super Special, 1908 Centaur Featherweight. 1988 Specialized Stumpjumper.