Why not just use the smaller ring on the cottered cranks?

Your question will likely better be answered over in classic and vintage, BTW. I know dirtdrop converted a PA10 to fixed gear (another Peugeot model with a cottered crank) and used the original cottered crank on his conversion, to nice effect.

I've converted several UO8s, but I've always switched out the cranks to cotterless in the process. (I've also done PXs and PRs). However, dirtdrop has the practical experience in this case. Post this over on C&V, he'll prolly be glad to help.