You won’t be able to put clincher type tires with tubes on rims designed for tubular tires. There is no way for a bead to seat on tubular rims, they were designed for the tires to be glued (or taped) down.

Your bike cleaned up nicely. You could try to source a Simplex SLJ rear derailleur, which is basically an all metal version of the Simplex Criterium you currently have on the bike. They are expensive, but do not require any mods to install.