As long as you clearly credit (who you believe to be) the writer, and tag any mp3s etc, no-one can accuse you of nefarious intent.

You have to distinguish between mechanicals (sales of hard copy) and performance royalties (radio/TV and streaming, though streaming is probably on a portal by portal basis): The only difficulty if money came your way is whether the writer's rights may have been divided by some contract back in the day: but that would be a nice problem to have, and probably wouldn't involve you unless you claimed any sort of share - so don't spend it all at once and keep the records.

There is no doubt a PRS notification portal; good luck with that, you'll probably have to pay them first.