I believe it's a Stronglight. A friend's '72 Raleigh came with one of these and it weighed a TON. It's an all steel cottered crank and, as AnthonyG said, there probably aren't, and maybe never were, replacement chainrings. They would be C&V items even if you could get them since these cranks haven't been made in decades.

So there are two choices:1) use it with the inner (42T?) chainring and leave the 52T in place or 2) replace the entire crank and bottom bracket.

You would do well to replace the entire crank and bottom bracket with modern, much lighter ones. What I don't know is the bottom bracket threading so you or your LBS would have to determine what it is. Is there anything engraved on the bb cups that would indicate it's threading?