I have an Sakae-made crank that I recovered from my sister-in-law's old Motobecane several years ago. It has the Motobecane name cast into the arms but the rear face has "SR" (Sakae's logo) molded in. Anyway, this crank is constructed just like your description and it is Al alloy, including the chainrings.
The outer chainring (52T) is a press-fit onto the crank arm hub and a flange of metal was turned over at the center to, in effect, rivet it in place. It cannot be removed or replaced. The inner ring (40T) is fastened to the outer ring with conventional looking bolts and probably could be replaced except the bolt circle diameter is something like 118 mm (Sheldon Brown says that is an obsolete SR size) and no commonly available replacement rings will fit.
If your crank is made like this one, you have no option but to replace the entire thing.